Printing device and printing method

ABSTRACT

When dot data indicating whether or not a dot is to be formed is formed based on the gradation value for each pixel forming an image, dots belonging to a plurality of pixel groups are printed in a common region in an overlapping manner, and the distribution of dots formed in the common region has a noise characteristic possessing a peak in the spatial frequency region on the high-frequency side. In a case where first and second pixels belonging, respectively, to two pixel groups are proximal pixels in the common region in a predetermined gradation range in which probabilities k1 and k2 at which a dot is formed in the first and second pixels are such that k1&lt;0.5 and k2&lt;0.5, a probability K of a dot being formed in both of the proximal pixels is set to be close to k1·k2.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation application of U.S. patent application Ser. No.13/428,528 filed on Mar. 23, 2012. This application claims priority toJapanese Patent Application No. 2011-065682 filed on Mar. 24, 2011,Japanese Patent Application No. 2011-088785 filed on Apr. 13, 2012 andJapanese Patent Application No. 2011-088789 filed on Apr. 13, 2011. Theentire disclosures of U.S. patent application Ser. No. 13/428,528 andJapanese Patent Application Nos. 2011-065682, 2011-088785 and2011-088789 are hereby incorporated herein by reference.

BACKGROUND

1. Technical Field

The present invention relates to a printing device and a printingmethod.

2. Background Technology

A technology for reproducing multi-gradation images has been used in aprinting device such as a printer in which one or more types of dots arerecorded on a printing medium. There have recently been remarkableadvances in multi-gradation technologies, and the formation of so-calledphotographic quality images can now be realized by combining two sizesof multi-colored dots such as cyan (C), magenta (M), yellow (Y), andblack (K) and controlling the distribution of these dots. When attemptsare made to reproduce a multi-gradation image with high image qualityusing fewer gradation values such as by forming or not forming dots(turning dots ON/OFF), controlling the distribution of dots properlybecomes a problem. Because of advances in technologies for analyzing thedistribution of these dots in a spatial frequency range, image qualitycan now be improved by maintaining in the dot distribution noisecharacteristics where the number of components at or below apredetermined frequency is kept as low as possible in a spatialfrequency range.

Blue noise characteristics are typical of these noise characteristics.Blue noise refers, for example, to characteristics in which the spatialfrequencies of an image formed uniformly of dots for reproducing animage having a constant gradation value include substantially nocomponents at or below a predetermined frequency. While the human eye issensitive to low-frequency characteristics below a certain level,high-frequency components are not very visible. For this reason, imageswith these blue noise characteristics have a smooth, high-quality feel.A well-known image formation technology having these blue noisecharacteristics has been disclosed in Patent Citation 1.

U.S. Pat. No. 5,341,228 (Patent Document 1) is an example of the relatedart.

SUMMARY

However, there are cases in which the highest quality images having bluenoise characteristics are those in which dots are formed properly in dotformation positions obtained using image processing. In actual printingdevices, with respect to the formation position of dots, dots can not beformed in the original formation position because of a variety offactors. For example, in inkjet printers which eject ink droplets fromnozzles, the landing positions of ink droplets during dot formation canbe different due to individual differences in each nozzle. Also, inprinters which form dots while the print head used to form dots movesrelative to a print medium such as printing paper, errors are known tooccur in dot formation positions due to positioning errors in the printhead. A typical error is an error that occurs in two-way printing inwhich dots are formed during forward action and during reverse action ofthe print head. Another similar well-known error is an error that occursin the multi-pass printing method in which a single raster is formed bymultiple passes in the main scanning direction.

A phenomenon is also known in which a shift occurs in the landingpositions of ink droplets, that is, in the formation positions of dots,due to buckling of the printing medium, such as printing paper absorbingink and buckling (“cockling”). A shift in the formation position of dotsdoes not occur only in printing devices using ink droplets. They occurin any type of printer that forms dots in a given region by dividingthem into a plurality of pixel groups, including thermal transferprinting devices, thermal sublimation printing devices, and so-calledline printers in which a print head is arranged in the width directionof the paper.

An advantage of the present invention is to solve at least some of theseproblems as realized in the embodiments and application examples below.

Application Example 1

A printing device for forming dots on a printing medium and printing animage, wherein

the printing device includes:

a dot data generating unit for receiving image data for an image to beprinted, and generating and associating with each pixel dot dataindicating whether or not a dot is to be formed on the basis of agradation value for each pixel forming the image, and

a printing unit for printing the image, when dots are to be formed onthe printing medium in accordance with the dot data, by dividing theformation of dots into a plurality of pixel groups having differentprinting conditions, and by performing at least a portion of the dotformation using the plurality of pixel groups in a common region;

the distribution of dots formed in the common region has a noisecharacteristic possessing a peak in the spatial frequency region on ahigher-frequency side relative to a low-frequency region at or below apredetermined spatial frequency; and

in a case where first and second pixels belonging, respectively, to twopixel groups among the plurality of pixel groups are proximal pixelsthat are near to each other in the common region in a predeterminedgradation range in which probabilities k1 and k2 at which a dot isformed in the first and second pixels are such that k1<0.5 and k2<0.5, aprobability K of a dot being formed on both of the proximal pixels isset to be close to k1·k2.

In the first application example, the distribution of dots formed in thecommon region has a noise characteristic possessing a peak in thespatial frequency region on the higher-frequency side relative to thelow-frequency region at or below a predetermined spatial frequency, theimage to be printed has characteristics close to so-called blue noisecharacteristics and/or green noise characteristics at least in thecommon region, and the image to be printed can be realized with highquality. In addition, the rise in the probability of dot formation inproximal pixels can be suppressed even in a case in which a shift occursin the formation positions of dots because the probability K of a dotbeing formed in both proximal pixels is set so as to be close to k1·k2.As a result, deterioration in image quality can be suppressed in a casein which a shift occurs in dot formation positions.

Application Example 2

The printing device according to the first aspect, wherein

the probability K of a dot being formed in both proximal pixels is setto be closer to k1·k2 in a case in which the size of a printing regionin the printing medium is equal to or greater than a first predeterminedvalue than in a case in which the size of a printing region in theprinting medium is less than the first predetermined value or a secondpredetermined value that is smaller than the predetermined value.

In the printing device of the second application example, a rise in theprobability of a dot being formed in a proximal pixel can be suppressedin a case in which the printing region on the printing medium is large,even in a case in which a shift occurs in the dot formation position. Asa result, it is possible to generate dot data in which the ratio atwhich dots are formed in adjacent pixels does not change significantlyin a case in which the printing region on the printing medium is large,even if a shift occurs in the dot formation position.

Application Example 3

The printing device according to the first or second aspect, wherein

whether or not to form a dot is decided by comparing the gradation valueof each pixel to each threshold value of a dither mask prepared inadvance; and

the probability K of a dot being formed in both proximal pixels is setto be close to k1·k2 in a case in which the threshold value whendetermining whether or not to form a dot in the common region has noisecharacteristics and the first and second pixels are proximal pixels thatare near to each other in the common region within a predeterminedgradation range.

In the printing device of the third application example, whether or notto form a dot is decided using a dither mask including the thresholdvalues mentioned above. As a result, the advantages of the dithermethod, such as high-speed determination for dot formation, can be fullyrealized, and deterioration in image quality can be suppressed in a casein which a shift in the formation position of dots occurs.

Application Example 4

The printing device in any of the first through third aspects, wherein

the printing unit performs a reciprocating action with respect to a mainscanning direction, and prints the image during both main scanning inthe forward action and main scanning in the reverse action; and

the first pixel group to which the first pixel belongs is a group ofpixels in which dots are formed by main scanning in the forward action,and the second pixel group to which the second pixel belongs is a groupof pixels in which dots are formed by main scanning in the reverseaction.

In the printing device of the fourth application example, a rise in theprobability of a dot being formed in a proximal pixel can be suppressedeven when a shift occurs in the position of dots formed by main scanningin the forward action and during main scanning in the reverse action.Therefore, the advantages of the reciprocating printing (bi-directionalprinting), such as shorter printing times, can be fully realized, andhigh quality can be maintained for images in which printing is performedin the common region.

In the printing device of the fourth application example, so-calledbi-directional printing can be performed, and the rise in theprobability of a dot being formed in a proximal pixel can be suppressed,even in cases in which a shift in the formation position of dots occursduring bi-directional printing. As a result, deterioration in imagequality can be suppressed in cases in which a shift in the formationposition of dots occurs.

Application Example 5

The printing device of the fourth aspect, wherein

the dots formed by main scanning in the forward action and the dotsformed by main scanning in the reverse action are arranged in analternating manner in both the main scanning direction and a secondaryscanning direction intersecting the main scanning direction; and

the proximal pixels are a combination of one pixel and another pixeladjacent to the pixel in the main scanning direction, and a combinationof a pixel and another pixel adjacent to the pixel in the secondaryscanning direction.

Application Example 6

The printing device of the fourth aspect, wherein

the dots formed by main scanning in the forward action and the dotsformed by main scanning in the reverse action are arranged in analternating manner in the main scanning direction, and are arranged sothat the dots formed by main scanning in the forward action or the dotsformed by main scanning in the reverse action are contiguous in asecondary scanning direction intersecting the main scanning direction;and

the proximal pixels are a combination of one pixel and another pixeladjacent to one side of the one pixel in the main scanning direction,and a combination of the one pixel and pixels adjacent to the adjacentpixel on either side in the secondary scanning direction.

Application Example 7

The printing device of the fourth aspect, wherein

the dots formed by main scanning in the forward action and the dotsformed by main scanning in the reverse action are arranged in analternating manner in a secondary scanning direction intersecting themain scanning direction, and are arranged so that the dots formed bymain scanning in the forward action or the dots formed by main scanningin the reverse action are contiguous in the main scanning direction; and

the proximal pixels are a combination of one pixel and another pixeladjacent to one side of the one pixel in the main scanning direction,and a combination of the one pixel and pixels adjacent to the adjacentpixel on either side in the main scanning direction.

In the printing devices of the fifth through seventh applicationexamples, it can be identified whether or not formation isolationcontrol has been performed on proximal pixels in any direction, in casesin which the arrangement of dots formed by main scanning in the forwardaction and main scanning in the reverse action is a so-called crossedarrangement (Application Example 5), an alternating column arrangement(Application Example 6), or an alternating raster arrangement(Application Example 7). In these combinations, any change in theprobability of dots being formed in adjacent pixels can be suppressed,and any deterioration in image quality caused by a shift in theformation position of dots can be suppressed, even in a case in which ashift in the position of dots formed by main scanning in the forwardaction and main scanning in the reverse action.

Application Example 8

The printing device in any of the first through third aspects, wherein

the printing unit forms dots while performing main scanning in the mainscanning direction, and prints the image by performing the main scanningoperation a plurality of times; and

the first pixel group to which the first pixel belongs and the secondpixel group to which the second pixel belongs are groups of pixels inwhich dots are formed during different main scanning operations amongthe main scanning operations performed a plurality of times.

In the printing device of the eighth application example, any change inthe probability of dots being formed in proximal pixels can besuppressed, and any deterioration in image quality caused by a shift inthe formation position of dots can be suppressed, even in a case inwhich a shift in the position of dots formed during different mainscanning actions occurs in so-called multi-pass printing.

Application Example 9

The printing device in any of any of the first through eighth aspects,wherein

the probability K is within the range k1·k2−0.2<K<k1·k2.

In the printing device of the ninth application example, in a case inwhich the average probability of a dot being formed in a given firstpixel and second pixel with a predetermined gradation value is k1, k2,the probability K of a dot being formed in both proximal pixels can bebrought sufficiently close to the probability k1·k2 in a case in whichthe dot formation probability has not been adjusted in accordance withthe dot arrangement. As a result, any degree of discrepancy in theformation position of dots can be addressed, and any deterioration inimage quality in these cases can be suppressed.

Application Example 10

The printing device in any of the first through ninth aspects, wherein

the predetermined gradation range is 0<k1<0.2, and 0<k2<0.2.

In the printing device of the tenth application example, the gradationrange in which the probability of dot formation in both proximal pixelsis brought close to k1·k2 is set on the low density side. As a result,any decline in graininess can be suppressed even when a shift in theformation position of dots occurs.

Application Example 11

The printing device in any of the first through tenth aspects,

Wherein

probabilities k1 and k2 are both k, and probability k is close to k².

In the printing device of the eleventh application example, theprobability of dot formation is handled in the same manner for the firstpixel group and the second pixel group. As a result, the process can besimplified.

Application Example 12

A printing method for forming dots on a printing medium and printing animage, wherein

the printing method includes:

a step for receiving image data for an image to be printed, andgenerating and associating with each pixel dot data indicating whetheror not a dot is to be formed on the basis of a gradation value for eachpixel forming the image, and

a step for printing the image, when dots are to be formed on theprinting medium in accordance with the dot data, by dividing theformation of dots into a plurality of pixel groups having differentprinting conditions, and by performing at least a portion of the dotformation using the plurality of pixel groups in a common region;

the distribution of dots formed in the common region has a noisecharacteristic possessing a peak in the spatial frequency region on ahigher-frequency side relative to a low-frequency region at or below apredetermined spatial frequency; and

in a case where first and second pixels belonging, respectively, to twopixel groups among the plurality of pixel groups are proximal pixelsthat are near to each other in the common region in a predeterminedgradation range in which probabilities k1 and k2 at which a dot isformed in the first and second pixels are such that k1<0.5 and k2<0.5, aprobability K of a dot being formed on both of the proximal pixels isset to be close to k1·k2.

In the twelfth application example, as in the first application example,dot formation can be controlled so that the distribution of dots formedin the common region has a noise characteristic possessing a peak in thespatial frequency region on the higher-frequency side relative to thelow-frequency region at or below a predetermined spatial frequency, andthe image to be printed has characteristics close to so-called bluenoise characteristics and/or green noise characteristics at least in thecommon region. As a result, the image to be printed can be realized withhigh quality using this data for dot formation. In addition, the rise inthe probability of dot formation in proximal pixels can be suppressedeven in a case in which a shift occurs in the formation positions ofdots because the probability K of a dot being formed in both proximalpixels is set so as to be close to k1·k2. As a result, deterioration inimage quality can be suppressed in a case in which a shift occurs in dotformation positions.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the attached drawings which form a part of thisoriginal disclosure:

FIG. 1 is a schematic block diagram of the printer 20 in an embodimentof the present invention;

FIG. 2 is a descriptive diagram illustrating the nozzle columns in theprint head 90 of this embodiment;

FIGS. 3A to 3C are descriptive diagrams showing a variation in which thedots formed during forward action and the dots formed during reverseaction are combined;

FIG. 4 is a flowchart of the printing process in this embodiment;

FIGS. 5A to 5C are descriptive diagrams showing dots formed duringforward action, dots formed during reverse action, and a combination ofthese dots;

FIGS. 6A and 6B are descriptive diagrams illustrating cases in whichthere is a shift in the dot formation position during forward action andduring reverse action;

FIGS. 7A and 7B are descriptive diagrams showing an arrangement exampleof dots and paired dots in a case in which a dispersion-type dither maskhas been used;

FIGS. 8A to 8C are descriptive diagrams showing adjacent pixels NR andND in relation to reference pixel OJ;

FIG. 9 is a graph showing the relationship between the dot incidence kand the paired dot incidence K;

FIG. 10 is a graph showing an example of a change in coverage in a casein which there is a shift in dot formation positions during forwardaction and during reverse action;

FIG. 11 is a graph showing the relationship between the amount of shiftin dot formation positions in pixel units and the deviation from k² ofthe paired dot incidence;

FIG. 12 is a flowchart showing the generating method for a paired pixelcontrol mask;

FIG. 13 is a descriptive diagram showing the relationship betweengradation values S and paired dot target values M;

FIG. 14 is a descriptive diagram showing an example of Visual TransferFunction (VTF) sensitivity characteristics;

FIGS. 15A to 15F are descriptive diagrams showing the relationshipbetween the reference pixel and adjacent pixels in another printingmethod;

FIGS. 16A to 16C are descriptive diagrams illustrating the dotdistribution during forward action, the dot distribution during reverseaction, and the dot distribution during synthesis;

FIG. 17 is a flowchart of the dither mask generating process in thesecond embodiment;

FIG. 18 is a descriptive diagram used to explain the arrangement ofstored elements for the reference pixel selection process;

FIG. 19 is a flowchart of the first dither mask evaluation processing;

FIG. 20 is a descriptive diagram showing an example of blue noisecharacteristics and green noise characteristics;

FIG. 21 is a descriptive diagram used to explain the processing in thethird embodiment;

FIG. 22 is a flowchart of the halftone processing in the thirdembodiment;

FIG. 23 is a descriptive diagram showing adjacent pixels in the thirdembodiment;

FIG. 24 is a flowchart of the result value setting process forcontrolling the number of paired dots in the third embodiment;

FIG. 25 is a schematic block diagram of the printer 20 in the fourthembodiment;

FIG. 26 is a flowchart of the printing process in the fourth embodiment;

FIG. 27 is a flowchart of the printing process in the fifth embodiment;

FIG. 28 is a flowchart of the dot data generating process in the fifthembodiment; and

FIG. 29 is a flowchart of the dither mask generating process in thesixth embodiment.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS A. First Embodiment

The first embodiment of the present invention will now be described.

A-1. Device Configuration

FIG. 1 is a schematic block diagram of the printer 20 in the embodimentof the present invention. The printer 20 is a serial-type inkjet printerwhich performs bi-directional printing. As shown, the printer 20includes a mechanism in which printing paper P is conveyed by a paperfeed motor 74, a mechanism in which a carriage 80 is caused to move in areciprocating manner by a carriage motor 70 in the axial direction of aplaten 75, a mechanism in which a print head 90 mounted in the carriage80 is driven, ink is ejected, and dot formation is performed, and acontrol unit 30 for governing the interaction of signals among the paperfeed motor 74, the carriage motor 70, the print head 90, and the controlpanel 99.

The mechanism for causing the carriage 80 to move in a reciprocatingmanner in the axial direction of the platen 75 includes a sliding shaft73 installed parallel to the axis of the platen 75 to slidably hold thecarriage 80, and a pulley 72 over which an endless drive belt 71 hasbeen stretched along with the carriage motor 70.

Ink cartridges 82-87 containing cyan ink (C), magenta ink (M), yellowink (Y), black ink (K), light cyan ink (Lc), and light magenta ink (Lm)are mounted in the carriage 80. Nozzle columns corresponding to each oneof these color inks is formed in the print head 90 at the bottom of thecarriage 80. When these ink cartridges 82-87 are mounted in the carriage80 from above, ink can be supplied from each cartridge to the print head90.

As shown in FIG. 2, nozzle columns are provided in the printing head 90so that a plurality of nozzles for ejecting ink droplets are arranged inthe secondary scanning direction. The arrangement pitch R of the nozzlecolumns is an integer multiple of the dot formation pitch (rasterdistance r). The printing is performed using so-called interlacing, inwhich each raster is completed during the printing process by repeatedlyperforming main scanning while moving the paper in the secondaryscanning direction relative to the print head 90 during each main scan.So-called overlap printing can also be performed in which one raster iscompleted over multiple main scans. As a result, printing can beperformed using so-called alternating column dot placement (FIG. 3A) oralternating raster dot placement (FIG. 3B) in which interlacing andoverlapping are combined to integrate dots formed in each raster or eachcolumn during either forward action or reverse action of the print head90. Alternatively, printing can be performed using so-called crossed dotplacement (FIG. 3C) in which dots formed during forward action and dotsformed during reverse action are arranged alternately in each raster andin each column. In the first embodiment, printing is performed usingcrossed dot placement as shown in FIG. 3C. Because the methods forrealizing the desired dot placement using interlacing and overlappingare well known, a detailed description has been omitted.

The control unit 30 for controlling the print head 90, carriage motor70, and paper feed motor 74 mentioned above to execute the printingprocess includes a CPU 40, a ROM 51, a RAM 52, and an EEPROM 60connected to each other via a bus. The control unit 30 deploys programsstored in the ROM 51 and the EEPROM 60 in the RAM 52 and executes themto control all of the operations of the printer 20. It also functions asthe dot data generating unit 42 and the printing unit 43 in the claims.The function units will be described in detail below.

A dither mask 62 is stored in the EEPROM 60. The size of the dither mask62 used in this embodiment is 64×64, and the threshold values from 0 to256 are stored in 4096 storage elements. Each threshold value is used inthe halftone processing described below. The placement of each thresholdvalue in the dither mask 62 is decided so as to give it characteristicsclose to those of a so-called blue noise mask. The characteristics ofthe dither mask 62 used in this embodiment are described in detailbelow. The configuration is one of a dispersion-type dither mask withhigh dispersion properties, which is similar to a dither mask used torealize high-quality images.

In this embodiment, the printing is performed by the printer 20 alone. Amemory card slot 98 is connected to the control unit 30, and image dataORG is read and inputted from a memory card MC inserted into the memorycard slot 98. In this embodiment, the image data ORG inputted from thememory card MC is data with three color components: red (R), green (G),and blue (B). The printer 20 performs printing using images ORG in thememory card MC. In addition, the printer can be connected to an externalcomputer via a USB port and/or LAN, and halftone processing can beperformed by the computer. The results can then be received and printedby the printer 20.

In a printer 20 having the hardware configuration described above, thecarriage motor 70 is driven to reciprocate the print head 90 relative tothe printing paper P in the main scanning direction, and the paper feedmotor 74 is driven to move the printing paper P in the secondaryscanning direction. The control unit 30 aligns the reciprocatingmovement of the carriage 80 (main scanning) with the feeding of theprint medium (sub-scanning), and drives the nozzles according to theappropriate timing on the basis of printing data to form ink drops ofthe appropriate color in the appropriate locations on the printing paperP. In this way, the printer 20 can print color images inputted from amemory card MC on printing paper P.

A-2. Printing Process

The printing process performed by the printer 20 will now be described.FIG. 4 is a flowchart showing the printing process performed by theprinter 20. Here, the user operates the control panel 99 to initiate theprinting process by performing a printing instruction operation for apredetermined image stored in the memory card MC. When the printingprocess has been initiated, the CPU 40 first reads and inputs theRGB-formatted image data ORG to be printed from the memory card MC viathe memory card slot 98 (Step S110).

When the image data ORG has been inputted, the CPU 40 references thelook-up table (not shown) stored in the EEPROM 60, and performs colorconversion on the image data ORG from the RGB format to the CMYKLcLmformat (Step S120).

When the color conversion has been performed, the CPU 40 performs aprocess (referred to below as halftone processing) as the dot datagenerating unit 42 in which the image data is converted into ON/OFF dotdata for each color (referred to below as dot data) (Step S130). In thisembodiment, this process is performed using the dither method. In otherwords, the inputted data is compared to the threshold values stored inthe storage elements constituting the dither mask 62 at the positionscorresponding to the inputted data. When the inputted data is greaterthan the threshold value, a determination is made to form a dot (dotON). When the inputted data is equal to or less than the thresholdvalue, a determination is made not to form a dot (dot OFF). The dithermask 62 used in this process is applied repeatedly in the main scanningdirection and the secondary scanning direction with respect to theinputted data aligned in the main scanning direction and the secondaryscanning direction. The halftone processing in this embodiment iscontrolled so that the generated dot data has predeterminedcharacteristics. The content of the control is dependent on the natureof the dither mask 62. The characteristics of the dither mask 62 aredescribed below. The halftone processing is not limited to binary ON/OFFdot processing. It can also be multi-value processing such as ON/OFFprocessing of large dots and small dots. Also, the image data providedin Step S130 can be obtained from image processing such as resolutionconversion processing and smoothing processing.

When the halftone processing has been performed, the CPU 40 performsoverlapping and interlacing alternatingly aligned with respect to dotdata to be printed in a single main scanning unit, harmonized with thenozzle arrangement of the printer 20, the paper feed rate, and otherparameters (Step S140). When overlapping and the interlacing areperformed, the CPU 40 drives the print head 90, the carriage motor 70,and the motor 74 as a process of the printing unit 43, and executesprinting (Step S150).

The following is a description of the arrangement of dots formed in thisprinting process. As is clear from the description provided above, theprinter 20 forms dots by ejecting ink from the print head at a pluralityof different timings (in other words, forward action and reverse action)in the common print region of the print medium while changing the inkejection position with respect to the print medium, and a printed imageis outputted in which the dots formed during the forward action(referred to below as the forward action dots) and the dots formedduring the reverse action (referred to below as the reverse action dots)are aligned with each other. Because the dots in the first embodimenthave a crossed arrangement (FIG. 3C), the dots formed during forwardaction by the print head 90 are formed in pixel positions arranged in analternating manner as indicated by the cross-hatching in FIG. 5A, andthe dots formed during the reverse action by the print head 90 areformed in pixel positions arranged in an alternating manner shifted onepixel in the column direction with respect to the dot positions duringforward action as indicated by the shading in FIG. 5B. The grouping ofpixels corresponding to the dots formed during forward action is calledthe first pixel group, and the grouping of pixels corresponding to thedots formed during reverse action is called the second pixel group. InFIG. 5A and FIG. 5B, the dots that are actually formed are indicated byblack circles and hatched white circles, respectively. The dot size isset to be larger than the diagonal size of the pixels so that thesurface of the print medium can be covered 100% at maximum density, evenwhen there is some discrepancy in dot formation positions. In theprinted image, the dots formed in the first and second pixel groups arealigned as shown in FIG. 5C.

Because the printing conditions for dot formation are different duringforward action and reverse action, the dots that are actually formed candiffer from those shown in FIG. 5C. For example, when the dot formationposition during forward action is shifted approximately one pixel in theraster direction (main scanning direction) with respect to the dotformation position during reverse action, the dots formed during forwardaction in the example shown in FIG. 5C are shifted in the main scanningdirection as shown in FIG. 6A. As a result, the area with overlappingdots is increased. When the shift is increased to two pixels as shown inFIG. 6B, the area with overlapping dots is increased even further.Without any shifts, as shown in FIG. 5C, there is very little overlapbetween dots. This is because the dots are separated as much as possibleand arranged using a dither mask having blue noise characteristics. Bycontrast, when a shift occurs in the dot formation position duringactual printing, the amount of overlap increases between the dotsbelonging to the first pixel group formed during forward action and thedots belonging to the second pixel group formed during reverse action asshown in FIG. 6A and FIG. 6B. When the amount of dot overlap increases,the coverage, which indicates the percentage of printing paper P coveredwith dots, changes. Dots that are not adjacent to each other when thereis no shift in the dot formation position can be formed in adjacentpositions when there is a shift in the dot formation position. In thiscase, there is no change in coverage, but the sense of graininess ischanged because dots are close to each other.

A-3. Halftone Processing

With these points in mind, the characteristics of the halftoneprocessing in the first embodiment will now be described. In the firstembodiment, the halftone processing indicated as Step S130 in FIG. 4decides whether or not a dot is to be formed in a given pixel positionby comparing the gradation values for the pixel belonging to the firstpixel group and the pixel belonging to the second pixel group to thedither mask 62 stored in the EEPROM 60. Data indicating the decision onwhether a dot is to be turned ON or OFF is called dot data.

As already described, the dither mask 62 used to generate dot data isset to have high dispersion properties. Thus, the placement of dots in alow density region of the image is sparse. From the standpoint ofdispersion properties, hardly any dots are placed in two pixels adjacentto each other vertically or horizontally. How this looks is shown inFIG. 7A. The example showing in FIG. 7A has an 8×8 region, and thegradation values for the image are a uniform 26/255. In this case, dotsare formed in approximately one-tenth of the pixels in the 8×8 region,or in approximately six pixels.

By contrast, in the first embodiment, the threshold values in the dithermask 62 have been set so that there is a significant probability thatdots will be placed in adjacent pixels. FIG. 7B shows an example inwhich dots are formed in adjacent pixels. In the first embodiment, thedither mask 62 is created so that there is a significant probabilitythat dots will be formed in adjacent pixels even in a region in whichthe gradation values for the pixels are low (for example, a regionhaving gradation values of 1-127/255).

Here, a significant probability is a probability set in the followingmanner. In the dither mask 62 used in the first embodiment, when thereis a probability of dot placement in each pixel belonging to the firstand second pixel groups of k (0<k<1) in an image data gradation valuerange of 0-127/255, the probability K of a dot being formed in a pixeladjacent to one in which a pixel has been formed rightward in the rasterdirection (main scanning direction) or in a pixel adjacent to one inwhich a pixel has been formed downward in the column direction(secondary scanning direction) is approximately 0.8×k².

Among the pixels adjacent to a reference pixel, the ones belonging to adifferent group in which dots are formed are referred to below asadjacent pixels. In the crossed arrangement shown in FIG. 3C, there arefour pixels adjacent to the reference pixel in the vertical andhorizontal directions. When a significant discrepancy occurs in theformation position of dots, it is between dots formed during the forwardaction and dots formed during the reverse action. Therefore, the dotoccurrence probability is adjusted not simply with respect to dots beingadjacent to each other, but with respect to dots from different pixelgroups being adjacent to each other. In the first embodiment, the dotsformed during forward action and the dots formed during reverse actionare arranged in an alternating manner, as shown in FIG. 3C. Thus, pixelsbelonging to a different group are adjacent to a reference pixel in fourlocations: up, down, left, and right relative to the reference pixel. Inthis embodiment, only the pixel to the right of the reference pixel inthe raster direction (main scanning direction) and the pixel underneaththe reference pixel in the column direction (secondary scanningdirection) are adjacent pixels of the reference pixel. This is becausepaired dots (dots formed in both adjacent pixels) can be counted usingonly one of two adjacent pixels symmetrical with respect to thereference pixel. In all of the pixels forming an image, the referencepixel moves successively from the upper left to the lower right of theimage. When only one of two adjacent pixels symmetrical with respect toa reference pixel is counted, all of the paired dots can be countedwithout duplication.

In FIG. 8A, the position of reference pixel OJ is (0,0). When there ispositive movement in the main scanning direction and the secondaryscanning direction, position (1,0) indicates adjacent pixel NR to theright, and position (0,1) indicates adjacent pixel ND below. In a casein which a relationship between the reference pixel OJ and eitheradjacent pixel NR or ND is identified, the combination is referred to aspaired pixels. In the first embodiment, as mentioned above, an adjacentpixel constituting paired pixels with the reference pixel is limited tothe pixels NR, ND to the right or underneath the reference pixel OJ.However, this can be reversed and the counting of paired dots can belimited to pixels to the left of the reference pixel OJ and above. InFIG. 8A, paired pixels are limited to pixels adjacent to the referencepixel. However, pixels whose probability of occurrence is considered toconstitute paired pixels do not have to be limited to adjacent pixels.As shown in FIG. 8B and FIG. 8C, pixels in positions set apart from thereference pixel can be considered to constitute adjacent pixels. Thesecases will be described in greater detail below.

The probability of dots being formed in paired pixels will now bedescribed. Here, gradation values correspond to the probability of a dotbeing turned ON (formed). When a halftone-processed image ORG is animage with a uniform gradation value of 26/255, approximately one dot isplaced for every ten pixels (k=0.1). By contrast, the probability K ofdots being formed in paired pixels using the dither mask in the firstembodiment is approximately K=0.8×k^(2≈0.008). In a conventional dithermask having high dispersion properties, the dispersion properties ofdots in a low density region takes precedence. The probability of dotsbeing formed in adjacent pixels or paired pixels that are adjacent is asclose to zero as possible. When a dither mask having the knowncharacteristics of a blue noise mask is used, no examples of dots formedin paired pixels at a gradation value of 26/255 have been found.

By contrast, in the first embodiment, the gradation values are0-127/255. In other words, the probability K of dots being formed inpaired pixels is approximately 0.8×k² in a dot formation probability krange of approximately 0-0.5. For example, when the gradation value is52/255 (k≈0.2), the probability K of dots being formed in paired pixelsis 0.032. In other words, dots are formed at an approximate percentageof three groups per 100 groups of paired pixels.

The percentage of dots formed in paired pixels is illustratedschematically in FIG. 9. In FIG. 9, the horizontal axis indicates theprobability of a dot being formed in a pixel, which corresponds to thegradation value of the image. In FIG. 9, the vertical axis indicates thepercentage of dots formed in paired pixels. In FIG. 9, solid line JD1indicates a case in which halftone processing was performed using thedither mask of this embodiment, and dotted line BN1 indicates a case inwhich halftone processing was performed using a blue noise mask. Dashedline WN1 indicates a case in which halftone processing was performedusing a white noise mask. Here, the white noise mask is a dither mask inwhich each threshold value was set using random numbers in asufficiently large mask size to obtain results similar to the randomdithering method in which the threshold value is generated each timeusing a random number. In contrast to a blue noise mask which has bluenoise characteristics that do not contain any low-frequency components,a white noise mask has white noise characteristics that evenly containeverything from low-frequency components to high-frequency components.

When a blue noise mask is used, as shown, the probability of dots beingformed in paired pixels is close to zero in a region having a low imagegradation value (gradation value 0 to 51, dot occurrence probability k=0to 0.2). By contrast, when a white noise mask is used, the formationposition for dots is random. The probability of dots being formed inpaired pixels closely matches k², where the probability of dot formationis k. While the dither mask employed in the present embodiment is adispersion-type dither mask in contrast to these characteristics, theprobability K of dots being formed in paired pixels is approximately0.8×k² in a gradation value range of 0 to 127 (dot occurrenceprobability k=0 to 0.5) as indicated by solid line JD1. In other words,the dither mask used in this embodiment has dispersion properties closeto those of a blue noise mask in the distribution of formed dots, butthe probability K of dots being formed in paired pixels is close to thatof a white noise mask. How a dispersion-type dither mask is created withan increased percentage of dot formation in paired pixels will bedescribed below in further detail.

A-4. Effect of the Embodiment

In a printer 20 of the first embodiment with the configuration describedabove, image data ORG is received, processing is performed as shown inFIG. 4 by the control unit 30, and an image is printed on printing paperP. At this time, halftone processing is performed on the gradation valueof each pixel constituting the image using the dithering method with adither mask 62 to convert them to the distribution of dots. Dispersionproperties take precedence in the dither mask 62, which is adispersion-type dither mask with characteristics typical of a blue noisemask. As a result, the image quality of an image produced by thishalftone processing and expressed by the dot distribution has a lowsense of graininess and high reproducibility of the image.

In addition, the probability of dots being formed in paired pixels, thatis, a pixel in the first pixel group to which dots formed during theforward action of the print head 90 belong, and a pixel in the secondpixel group to which dots formed during the reverse action belong whichhave an adjacent relationship, is set so as to be higher than that of ablue noise mask. Therefore, it has characteristics by which imagequality is less likely to deteriorate even when there is a shift in theformation position of dots during forward actions and reverse actions.This point will be described with reference to FIG. 10.

FIG. 10 is a graph showing simulation results for a change in coveragein a case in which image data ORG was processed with gradation values inwhich the percentage of dot formation was 96/255. In this figure, thehorizontal axis indicates the shift in the formation position of dotsduring forward action and reverse action. Here, the units are pixels.The vertical axis indicates the change in coverage. In the graph shownin FIG. 10, the solid line JE1 indicates a case in which the dither mask62 in the first embodiment is used, and dashed line BB1 indicates a casein which a typical blue noise mask is used. This mask was created sothat dots are generated as discretely as possible. Here, coverage meansthe percentage of paper P covered by formed dots, and the change incoverage means the change in the percentage of paper P covered byoverlapping dots caused by a shift in the formation position of dotsrelative to the percentage of paper covered by dots in the original casein which there is no shift in the formation position of dots.

In FIG. 10, the dot size is set to be somewhat larger than the pixelsize in order to approximate actual printing conditions with theprinter. As a result, even when dots overlap, dot overlapping occurs inwhich adjacent dots contact each other, and coverage is reduced. In atypical blue noise mask, the dispersion placement is such that thedistance between dots is as far apart as possible. As a result, contactbetween pixels, the cause of coverage reduction, is minimized when thereis no shift. Therefore, when a shift occurs in the formation position ofdots during reciprocal printing with an actual printer 20, the formationposition of dots slips from the optimal placement, contact between dotsand overlapping increase, and coverage is generally reduced. In a casewhere data of the same gradation value is printed, a change in coveragewill result in variation in the concentration of the image as well as adrop in the image quality. Unevenness in image quality due to changes incoverage is more conspicuous in larger printed items printed by largeprinters. This is because larger printed items are generally viewed froma greater distance. When viewed from a greater distance, uneven printingat lower frequencies is more noticeable.

When the dither mask 62 in this embodiment is used, as shown in FIG. 10,it is evident that there is less change in coverage compared to a casein which an ordinary dispersion-type dither mask is used, even whenthere is a shift in the formation position of dots during forward actionand reverse action. FIG. 10 shows the change observed when the amount ofshift Δd in the printing position during reverse action relative to theprinting position during forward action is greater when M is an oddnumber (i.e., 1, 3 . . . ) than when Δd is an even number (i.e., 2, 4 .. . ), where Δd is expressed in pixel units, and a period is a shift of2. The reason for the change in a period of a shift of 2 is that theposition of dots formed during forward action and the position of dotsformed during reverse action completely overlap when the horizontalshift is an odd number during printing using crossed arrangement asshown in FIG. 3C. In a simulation in which a shift in the formationposition of dots due to other factors was not taken into account, andprinting was performed under the assumption that the shift in theformation position of all dots was the same during forward and reverseaction, the reduction in coverage became apparent when the horizontalshift was an odd number, as shown in FIG. 10. In an actual printer 20, asmall position shift in pixel units is superimposed on the shift in theformation position of dots during reciprocating printing. As a result,the change in coverage shown in FIG. 10 is made flatter. In a case inwhich the dither mask 62 in this embodiment is used [as indicated by]the solid line JF1 in FIG. 10, the change in coverage is flatter still,and hardly poses a problem. By contrast, the change in a case in which adispersion-type dither mask is used [as indicated by] the dashed lineBB1 in FIG. 10 is flatter to some degree but not sufficientlyeliminated, so that a change in coverage remains.

Therefore, a printer 20 using the dither mask of this embodiment cansuppress a decline in printing quality caused by a shift in theformation position of dots during reciprocating action better than aconventional dispersion-type dither mask, and can realize better printquality. Even in a case in which a shift in the formation position ofdots occurs when an image with low gradation values is printed, thenumber of paired dots formed does not change very much. This is becausethe percentage of paired dots formed in the low gradation value regionoriginally is close to the percentage k² occurring in a case in whichrandom dot placement is employed. As described using FIG. 9, when theprobability K of dots being formed in paired pixels in the low gradationvalue region is small or close to zero as in a blue noise mask, and alarge shift occurs in the formation position of dots in the forwardaction and reverse action, or in the formation position of dots aftersub-scanning, dots that were not intended to be paired originally becomeadjacent to each other or overlap, the change in coverage is large, anduneven density occurs in the image. The sense of graininess in the lowdensity region also deteriorates. In halftone processing using thedither mask of this embodiment, the probability of dots being formed inpaired pixels is increased from the start, so the probability of dotsbeing formed in paired dots does not change very much even when there isa shift in the formation position of dots. As a result, this does notlead to deterioration in the sense of graininess in the image.

Setting the percentage of paired dot formation close to k² is based onthe following new finding. When there is a shift in the formationposition of dots in a certain pixel group and the shift is sufficientlylarge, it has been found that the probability of dots becoming adjacentto each other in a certain direction and becoming paired dots convergeson k², even when the interval between dots is increased as much aspossible through dispersed placement using a blue noise mask. Actualblue noise masks were studied and it was found, as shown in FIG. 11,that the probability of paired dots occurring converged on the constantvalue k² when the shift was from four to five or more pixels. This isbecause two pixels originally placed apart from one another becameadjacent when the shift was large. Because the correlation for thepresence or absence of dot formation for both pixels is reduced when thedistance between two pixels is sufficient, the probability of dots beingformed simultaneously in both pixels is k², which is obtained by simplymultiplying the gradation values (dot formation probability k) of thetwo. Therefore, if the paired dot incidence in a situation with no shiftis brought close to k² in advance, the paired dot incidence will notchange very much no matter how much of a shift occurs, and any change incoverage can be suppressed.

In the first embodiment, the probability K of dots being formed inpaired pixels is K=0.8×k². Therefore, a reduction in the dispersionproperties of the dot distribution in a case in which there is no shiftin the formation position of dots can be suppressed. This coefficientadjusts the probability of paired dots occurring. When the coefficientis 0.8, it means the incidence of paired dots has been suppressed to80%. The coefficient can be set anywhere within a range, for example,from 0.6 to 1.4. When the coefficient is set in a range from 0.8 to 1.2,a change in the probability of paired dots occurring relative to a shiftin the formation position of the dots can be favorably suppressed. Acoefficient closer to 1.0 is desirable from the standpoint ofsuppressing any change in the probability of paired dots occurring. Whenthe dispersion properties of the dots in a low gradation region takesprecedence, the coefficient can be adjusted to 0.8 or less, for example,from 0.6 to 0.8.

A-5. Dither Mask Generating Method

The dither mask used in the first embodiment is generated using thefollowing method. FIG. 12 is a flowchart showing an example of thegenerating method for the dither mask used in the first embodiment. Inthis embodiment, a blue noise mask is prepared, and a dither mask isgenerated from this blue noise mask in which the probability of dotsbeing formed in paired pixels approaches K². The generated dither maskis called a “paired pixel control mask” below. While the mask is beinggenerated, it is referred to as a “working mask”.

When a paired pixel control mask is generated, a blue noise mask isfirst prepared (Step S200). In this example, a 64×64 blue noise mask isused. The blue noise mask in this example has 255 threshold values from0 to 254 stored in a 64×64 matrix. Next, a process is performed on theworking mask in which the number of paired dots per gradation value iscounted in the entire gradation range (Step S210). More specifically,adjacent right paired dots RPD [1, 2, . . . 127] and adjacent underneathpaired dots UPD [1, 2, . . . 127] are counted individually. In thefollowing description, the use of parentheses such as (S) indicates thevalue for a gradation value S, and the use of brackets such as [a, . . .x] indicates the sequence a-x in a gradation range. The sequence a-x ina gradation range is also expressed as [a:x].

Because all of the threshold values in the working mask are known, theformation position of dots for each gradation value can be examined inthe gradation value 1-127/255 range. As a result, the number of adjacentright paired dots RPD (S) and adjacent underneath paired dots UPD (S)can be easily counted for each gradation value S. Here, the number ofpaired dots counted is limited to gradation values 1-127/255 because thepaired pixel control mask used in the first embodiment, that is, a maskhaving predetermined characteristics for the probability of paired dotsoccurring in the 1-127/255 gradation range, is generated. When thegradation values S are larger, the number of paired dots approaches theattempted probability even in a blue noise mask. Therefore, instead ofcounting the number of adjacent paired dots in the entire range, theprobability of paired dots occurring can be adjusted in the gradationvalue 1-127/255 range as already described with reference to FIG. 9.However, the method described below can also be applied to a case inwhich the number of paired dots is counted in the entire gradation rangeto adjust the probability of occurrence.

After counting the number of adjacent right paired dots RPD[1:127] andthe number of adjacent underneath paired dots UPD[1:127] in apredetermined gradation range (1-127/255 here) in Step S210, it isdetermined whether or not the number of paired dots for each gradationvalue S is within the target range M(S) (Step S220). Here, the targetrange M(S) is set in the following way. If the dither mask has whitenoise properties, dots are generated randomly, and the probability of adot being formed in a pixel is k. In this case, the probability of a dotbeing formed in an adjacent right pixel or adjacent underneath pixel(the probability of paired dots occurring) is k² in both cases. When thegradation value of the image is 1, k=0.00392156 (=1/255), and theprobability of paired dots being generated is k²=0.0000154. Therefore,in a case in which it is assumed that dots are formed randomly, thevalue H predicting the presence of paired dots among 64×64 pixels(referred to as the prediction value below) is H=k²×4096=0.126≈0. Thiscalculation is repeated in advance in the 1-127/255 gradation valuerange to determine the theoretical prediction values for paired dotsH[1:127], and this is multiplied by coefficient 0.8 to obtain the paireddot target values m[1:127] for each gradation value S. In thisembodiment, the target value m(S) is given a ±20% range, and this iscalled the target range M(S).

FIG. 13 shows the paired dot prediction values H[1:32] and the targetvalues m[1:32] for a case in which the gradation values S are 1-32. Asshown in this figure, gradation value S=10 has prediction value H(10)=6and target value m(10)=5, and gradation value S=20 has prediction valueH(20)=25 and target value m(20)=20.

In Step 220, the theoretical paired dot target range M[1:127] determinedin this manner is compared to the number of adjacent right paired dotsRPD[1:127] and the number of adjacent underneath paired dots UPD[1:127].In a case in which it is determined as a result of the comparison thatthe number of paired dots RPD[1:127] and URD[1:127] is not within thetarget range M[1:127], a process is performed in which the appropriatenumber of threshold values (for example, two threshold values) arerandomly replaced among the threshold values in the working mask (StepS230). Since the threshold values are randomly replaced, then as long asthreshold values corresponding to the same pixel group are replaced,replacing between different pixel groups can also take place.

Because threshold values in the working mask are replaced, the number ofpaired dots in each threshold value changes. Therefore, the number ofpaired dots is modified due to the replacement of threshold values (StepS240). Because the number of paired dots only changes within thegradation value range corresponding to the replaced threshold values, arecount is not performed in gradation range 1-127/255. Instead, theadjacent right paired dots RPD[p:q] and the adjacent underneath paireddots UPD[p:q] are recounted when, for example, threshold value p andthreshold value q (p<q) have been replaced. While the replaced thresholdvalues are selected randomly, the paired dot generating characteristicshave to be adjusted in gradation value range 1-127/255. Therefore, atleast one of the replaced threshold values should preferably be athreshold value within this range.

The number of paired dots recounted in this manner is examined, and itis determined whether or not the paired dot characteristics have beenimproved (Step S250). Here, whether or not the paired dotcharacteristics have been improved is determined in the followingmanner.

(A) An improvement is determined when, as a result of threshold valuereplacement, the number of adjacent right and underneath paired dotsRPD[p:q], UPD[p:q] is closer to k².

(B) An improvement is determined when, as a result of threshold valuereplacement, either the number of adjacent right or underneath paireddots RPD[p:q], UPD[p:q] is closer to k² and the other number hasremained unchanged.

(C) An improvement is determined when, in a case in which there is someimprovement and some deterioration in gradation range [p:q], the sum ofthe difference between the number of paired dots generated by eachgradation value in the gradation range and the prediction value for eachgradation value is smaller.

In a case in which this determination is performed and it is determinedthat there has been no improvement in the paired dot characteristics,the process returns to Step S230, and the process is executed againbeginning with the random replacement of threshold values. When twothreshold values are replaced in the threshold value replacementprocess, the number of combinations within the entire gradation range is₄₀₉₆C₂. The number is ₂₀₄₈C₂ when the range is limited to the gradationrange 1-127/255. Therefore, while there is a considerable number ofpossible threshold value replacement combinations and a considerableamount of time is required to exhaust all possibilities, a replacementcombination which improves the paired dot characteristics will be foundwhen this process is performed successively (YES in Step S250).

When it has been determined that the paired dot characteristics havebeen improved, it is then determined whether or not there is any problemwith graininess (Step S260). Here, no problems with graininess means thegraininess index shown below is within the target range, or the index isnot within the target range but there has been an improvement sincethreshold value replacement. Because the graininess index is well knownin the art (see, for example, Japanese Laid-open Patent Publication No.2007-15359), a detailed description has been omitted. However, this isan index obtained by performing a Fourier transform on an image todetermine the power spectrum FS, weighting the resulting power spectrumFS corresponding to the visual transfer function (VTF) or sensitivitycharacteristics relative to the spatial frequencies visible to humans,and integrating this at each spatial frequency. A VTF example is shownin FIG. 14. Various equations have been proposed as experimentalequations for providing this VTF. A typical experimental equation isshown in Equation (1). Variable L is the observation distance, andvariable u is the spatial frequency. The graininess index can becalculated using the calculation equation shown in Equation (2) on thebasis of the VTF. Coefficient is a coefficient for combining theresulting value with human vision. From the calculation method, it isclear that the graininess index can be said to indicate whether or notdots seem to stand out to humans. A lower graininess index is superiorfrom the standpoint of print quality because it means the dots are lessvisible.

$\begin{matrix}\left\lbrack {{Equation}\mspace{14mu} 1} \right\rbrack & \; \\{{{VTF}(u)} = {5.05\;{\exp\left\lbrack \frac{{- 0.138}\mspace{14mu}\pi\mspace{14mu}{Lu}}{180} \right\rbrack}\left\{ {1 - {\exp\left\lbrack \frac{{- 0.1}\mspace{14mu}\pi\mspace{14mu}{Lu}}{180} \right\rbrack}} \right\}}} & (1) \\\left\lbrack {{Equation}\mspace{14mu} 2} \right\rbrack & \; \\{{{Graininess}\mspace{14mu}{Index}} = {K{\int{{{{FS}(u)} \cdot {{VTF}(u)}}\ {\mathbb{d}u}}}}} & (2)\end{matrix}$

The blue noise mask prepared initially is configured so that thegraininess index has the lowest value. However, when threshold valuesare randomly replaced in Step S230, the graininess of the working maskis worse than the blue noise mask. Therefore, a target range is providedfor the graininess index, which is the acceptable range based oncharacteristics of human vision. This range is used to determine whetheror not there are any problems. Because the graininess index is a valuedetermined for each gradation value, an upper limit is prepared for eachgradation value. When the graininess index for each gradation value isbelow the upper limit, the graininess can be determined to be within thetarget range.

In a case in which there is a problem with graininess, that is, thegraininess is not within the target range, and it has been determinedthat there has been no improvement compared to the situation before thethreshold values were replaced (NO in Step S260), the process returns toStep S230 and is repeated beginning with the replacement of thresholdvalues. When, as a result of repeating the process from Step S230 toStep S260, it has been determined that the paired dot characteristicsare improved and that there is no problem with graininess (YES in bothStep S250 and Step S260), the loop from Step S230 to Step S260 isexited, the process returns to Step S220, and it is determined whetheror not the paired dot generation characteristics are within the targetrange.

When it cannot be determined whether the paired dot generationcharacteristics are within the target range (NO in Step S220), theprocess is repeated beginning with Step S230. In the process shown inFIG. 12, Step S220 through Step S260 are repeated by replacing thethreshold values until the conditions have been satisfied. When thenumber of times the process from Step S230 to Step S260 being performed(referred to below as the number of loops) is small, another acceptableprocess involves increasing the upper limit for the graininess in Step260, and bringing the upper limit close to the final target value as thenumber of loops increases. By changing the upper limit depending on thenumber of loops performed, the graininess index can be prevented fromdropping to the local minimum value.

When the loop from Step S230 to Step S260 has been performed severaltimes, and it has finally been determined that there are no problemswith graininess and that the adjacent right paired dots RPD[1:127] andthe adjacent underneath paired dots UPD[1:127] are within the targetrange M[1:127] (YES in Step S220), the paired pixel control mask hasbeen completed. At this time, the working mask is saved as the paireddot control mask (Step S270), the process is exited at END, and thepaired dot control mask generating routine (FIG. 12) is ended. In thisdescription, it was determined whether or not the paired dot generatingcharacteristics are within the target range using the 1-127/255 rangewithin the entire gradation value range in which paired dots can begenerated. However, any gradation range can be used by the paired dotcontrol mask to control the probability of paired dots occurring. Forexample, the range can be limited to the low density range (thegradation range corresponding to a dot generation probability of k=0 to0.25, 0.2 to 0.5).

A paired pixel control mask can be obtained on the basis of a blue noisemask using the method described above. In the first embodiment, thedither mask is a dither mask used to determine dot formation. Becausethe paired pixel control mask is based on a blue noise mask, it hardlypossesses any components in the low-frequency range to which humans havehigh visual sensitivity when the distribution of dots formed in theimage in the low gradation value range is analyzed in terms of spatialfrequencies. As a result, a dither mask can be provided which is able torealize high picture quality. In addition, using this paired pixelcontrol mask, the probability of paired dots occurring in adjacentpixels is approximately k²×0.8, where k is the probability of a dotbeing formed at each gradation value. As a result, a dither mask can beprovided in which the change in coverage is small even when there is ashift in the formation position of dots during forward action andreverse action, and in which uneven density in the image caused by ashift in the formation position of dots can be suppressed.

In this embodiment, the paired pixel control mask was generatedbeginning with a blue noise mask. However, it can also be generated froma dither mask having other characteristics. As mentioned above, the timerequired to generate a mask can be reduced if the dispersion propertiesare superior and the original dispersion properties are close to thecharacteristics to be converged upon, such as in a blue noise mask or agreen noise mask. When a dither mask is generated from one of these, apaired pixel control mask can be generated by applying the followingrules.

(1) The threshold values are placed in a matrix in successive order fromeither the small side or the large side.

(2) When the next threshold value is placed relative to a thresholdvalue already placed in a certain position, the placement position forthe next threshold value is associated with the evaluation value for thesituation using an evaluation value such as the graininess index. Inaddition, the candidate placement position for the next threshold valueis identified in successive order from the highest to the lowestevaluation.

(3) The candidates are taken out in successive order from the highest tothe lowest evaluation, and the number of paired dots in this situationis counted. When a candidate is found for the required number of paireddots (for example, the number shown in FIG. 12), the next thresholdvalue is placed in this location.

(4) Rules (1) through (3) are repeated until the threshold values areexhausted. The placement of threshold values can be decided from one,and a paired pixel control mask generated using these rules.

A-6. Modification Examples of First Embodiment

Modification examples of the first embodiment described above will nowbe described. In the first embodiment, the formation position of thedots alternated between the main scanning direction and the secondaryscanning direction during forward action and reverse action, and thepositions of the adjacent pixels constituting paired pixels, as shown inFIG. 8A, were two pixels, one to the right in the main scanningdirection and another underneath in the secondary scanning direction.However, adjacent pixels are not limited to these two. These adjacentpixels can be to the right and to the left in the raster underneath inthe secondary scanning direction. When the position of the referencepixel OJ is (0,0), a total of four groups of paired dots are counted inwhich the adjacent pixels are the pixels at the (1,0) and (0,1)positions, and the adjacent pixels are the pixels at the (−2,1) and(2,1) positions. This is shown in FIG. 8B. The range can be expandedfurther and enlarged to eight pixels as shown in FIG. 8C. If the pairedpixel range is expanded, the occurrence of density unevenness can besuppressed even when there is a shift in the formation position of dots.It is desirable to expand the range of adjacent pixels in a direction inwhich a shift in the formation position of dots does not occur duringprinting. As shown in FIG. 8B, setting the adjacent pixels broadly inthe main scanning direction is effective at suppressing density[unevenness] caused by a shift in the main scanning direction.

Dots formed during forward action [and] dots formed during reverseaction can alternate by column as shown in FIG. 3A and/or alternate byraster as shown in FIG. 3B. Even in this case, the adjacent pixel rangecan be set in many different ways. In the case of alternating columns,as shown in FIG. 15A through FIG. 15C, one, three or eight pixels can beset as adjacent pixels relative to the reference pixel OJ. In the caseof alternating rasters, as shown in FIG. 15D through FIG. 15F, one, fouror eight pixels can be set as adjacent pixels relative to the referencepixel OJ. In both of these cases, a paired pixel control mask can begenerated on the basis of a blue noise mask using the method shown inFIG. 12 in which the probability K of paired dots occurring is K=0.8×k².

In order to simplify the description of the embodiment and modificationexamples, the resolution of the gradation values in the image was set ateight bits, and the range of the threshold values was set at 0-255. Ifthe threshold values placed in the dither mask are set at 0-4095, andthe number of bits expressing the gradation values in the image isincreased, for example, to ten bits, the number of dots placed withrespect to the smallest gradation value 1 can be reduced, and the numberof dots increased with each increase of 1 in the gradation value can bereduced. As a result, control of the probability of paired dotsoccurring can be performed more precisely. When the size of the dithermask is increased to 128×128 or 256×512, the number of dots formed in acase in which the gradation value expressed using ten bits is 1 becomesfour in the case of the former and 32 in the case of the later. The sizeof the dither mask, the number of bits expressing the gradation values,and the type of threshold value placed in the dither mask can be decidedby taking into account the purpose of the halftone processing to beexecuted (image quality takes precedence, processing speed takesprecedence, large format printing is to be performed, and the like)and/or the processing time.

In the first embodiment, a paired pixel control mask was prepared inwhich paired dot control was performed using a gradation value range of0-127/255. In other words, paired dot control was performed so theprobability k of dots occurring was within the range 0<k<0.5. However,the upper limit can also be limited on the low gradation value side. Forexample, paired dot control can be performed in a range limited to0<k<0.2. Because the likelihood of dot overlap occurring due to a shiftincreases as the size of the dots increases relative to the size of thepixels, a gradation range in which density fluctuation becomes a problemis moved to a lower density. Therefore, it is realistic to change theadjusted range in accordance with the actual size of the dots relativeto the pixel size. Also, the lower limit can be limited on the highgradation value side. Generally, the problem of density unevenness isnot prominently manifested in the low gradation range near a gradationvalue of zero, even when the original dot formation positions are farapart and a shift in the formation position of dots occurs. Therefore, apaired pixel control mask can be generated in which the range is limitedto 0.1<k<0.4 or 0.2<k<0.5. Also, the percentage of dots formed duringforward action and the percentage of dots formed during reverse actionis changed from the initial percentages. Here, different dot formationprobabilities k1, k2 can be set for them when paired dot control isperformed.

B. Second Embodiment B-1. Dither Mask

The following is a description of the second embodiment of the presentinvention. The hardware of the printer 20 in the second embodiment isthe same as that of the first embodiment (see FIG. 1). The print controlprocess for the printer 20 (FIG. 4) is also the same except that adifferent dither mask is used in the halftone processing. The halftoneprocessing in the second embodiment also uses the so-called dithermethod.

The following is a list of the similarities and differences between thedither mask used in the second embodiment and the dither mask used inthe first embodiment.

(1) Similarities:

The size of the dither mask is 64×64 in both.

The probability K of paired dots occurring is set in both to K=0.8×k²,where k is the probability of dot formation.

A dispersion-type dither mask is used in both which gives priority tothe dispersion properties.

(2) Differences:

In the first embodiment, the threshold values for the 64×64 dither maskwere created to take into account only the dispersion properties of thedot distribution formed during forward action and reverse action of theprint head 90. By contrast, the dither mask in the second embodiment iscreated to take into account the dispersion properties of dots in thefirst pixel group to which the dots formed during forward action of theprint head 90 belong and the dots in the second pixel group to which thedots formed during reverse action of the print head 90 belong.

These differences will now be described. An image is formed by theprinter 20 from dots formed during forward action of the print head 90and dots formed during reverse action of the print head. Therefore, thedistribution of dots in an image obtained using certain gradation valuesis a distribution of dots formed during both the forward action andreverse action of the print head 90. Conventionally, the thresholdvalues for the dither mask are decided while targeting higher dispersionproperties for the dots in this case. By contrast, the dither mask usedin the second embodiment takes into account the dispersion properties ofdots in the first pixel group to which the dots formed during forwardaction of the print head 90 belong and the dots in the second pixelgroup to which the dots formed during reverse action of the print head90 belong. In other words, the dither mask used in the secondembodiment, as shown in FIG. 16, takes into account the dispersionproperties of the dots themselves formed during forward action (FIG.16A) and the dispersion properties of the dots themselves formed duringreverse action (FIG. 16B) when a particular image is formed. Needless tosay, in addition to this, the dither mask used in the second embodimentcan be said to have a probability K of paired dot occurrence of 0.8×k².A method for generating such a dither mask is described below, but anymethod able to generate a dither mask with these characteristics can beused.

A printer 20 of the second embodiment using a dither mask with thesecharacteristics is able to sufficiently suppress deterioration in imagequality even when a shift occurs in the position of dots formed duringthe forward action and reverse action of the print head 90. The reasonsare as follows.

(i) In the second embodiment, a dither mask is used which takes intoaccount the dispersion properties of the first pixel group to which dotsformed during forward action of the print head 90 belong and the secondpixel group to which dots formed during reverse action of the print head90 belong. In this way, because the dispersion properties of the dotsbelonging to both groups are guaranteed, even when a shift occurs in theformation position of dots during forward action and reverse action, adecrease in the dispersion properties of the dots in a case in which thedots of both groups overlap in the common region remains very limited.This is because the graininess in a case in which the dots belonging tothe two pixel groups are combined indicates a strong correlation withthe graininess of the individual dots belonging to each pixel group.

(ii) In the second embodiment, the probability K of paired dotsoccurring is 0.8×k². Therefore, as described in the first embodiment,any change in coverage is suppressed or a significant change in theprobability of paired dots occurring does not occur even when a shiftoccurs in the position of dots formed during forward action and reverseaction of the print head 90. As a result, deterioration in print qualityis suppressed even when a shift in the formation position of dotsoccurs.

B-2. Dither Mask Generating Method

The steps in the generation method for a dither mask 62 with thesecharacteristics are shown in FIG. 17. As shown, when generating a dithermask 62, threshold values are first prepared based on the size of thedither mask 62 (Step S310). In the second embodiment, the size of thedither mask 62 is 64×64. However, the description has been simplified byusing an 8×8 sized mask with 64 storage elements. In Step S310,threshold values 0-63 are prepared. In other words, a threshold value isprepared for each storage element.

When the threshold values have been prepared, the target threshold valueselection process is performed (Step S320). In the target thresholdvalue selection process, a threshold value is selected as a targetthreshold value among the prepared threshold values 0-63 not yet storedin a storage element. In this embodiment, the target threshold valuesare selected from among the prepared threshold values in successiveorder from the smallest to the largest. As shown in FIG. 18, in a casein which threshold values 0-3 have already been stored in storageelements constituting the dither mask by performing the steps describedbelow, the target threshold value selected in the next Step S320 isvalue 4.

When the target threshold values have been selected, the first dithermask evaluation process is performed (Step S400). In the first dithermask evaluation process, in a case in which a target threshold value isto be stored in a storage element that is not yet storing a preparedthreshold value (referred to below as an empty storage element), anevaluation value E1 is calculated for each empty storage element whichindicates the extent of dot dispersion in a dot formation patternindicating the arrangement of storage elements in which threshold valueshave already been stored (referred to below as decided storageelements). The calculation method for this evaluation value E1 isdescribed below. However, in this embodiment, a smaller evaluation valueE1 indicates better dot dispersion properties and is good from thestandpoint of the graininess of the printed image.

Next, the stored element is decided using the evaluation value E1 (StepS330), and it is determined whether or not the deciding process has beencompleted for all of the storage elements (Step S340). When the processhas not been completed for all of the storage elements, the processreturns to Step S320, and the process described above is repeated.

When the first dither mask evaluation process described above has beenperformed and threshold values have been stored in all of the storageelements (YES in Step S340), the temporary dither mask has beencompleted. Next, the second dither mask evaluation process is performed(Step S500). The second dither mask evaluation process is equivalent tothe process performed in the first embodiment in which the paired pixelcontrol mask is generated. When the paired pixel control mask in thefirst embodiment is generated, the process starts with a blue noise mask(FIG. 12, Step S200). In the second dither mask evaluation process inthe second embodiment, the process starts with the temporary dither maskobtained from the first dither mask evaluation process. The dither maskused in the second embodiment is generated in the manner describedabove.

B-3. First Dither Mask Evaluation Process

Using FIG. 19, the following is a description of the first dither maskevaluation process in the dither mask generating process describedabove. In the first dither mask evaluation process, a grouping processis performed first as shown in FIG. 19 (Step S410). In the groupingprocess, the plurality of storage elements constituting the dither maskare divided into a plurality of groups, focusing on whether thethreshold values stored in the plurality of storage elements form dotsin the dot formation positions applied in the halftone processing duringforward action or reverse action. In other words, groups of storageelements are set on the basis of the placement mode for dots formedduring forward action of the print head and dots formed during reverseaction of the print head (in the second embodiment, the alternatingcolumn mode shown in FIG. 3A is used). The groups are set on the basisof different timings in a case in which the ink ejection position ischanged relative to the print medium in the common printing region ofthe print medium and ink is ejected from the print head to form dots ona plurality of different timings. Instead of, or in addition to, forwardaction and reverse action, main scanning can be performed successively(main scanning can be performed several times) on a plurality ofdifferent timings in a case in which dots are formed in a commonprinting region N times (N being three or more times) in the mainscanning direction.

When the grouping process has been performed, the dots in the decidedstorage elements are turned ON (Step S420). In FIG. 18, the singlehatching indicates the dots in the decided storage elements which arestoring threshold values 0-3 and which have been turned ON. When thedots in the decided storage elements have been turned ON, the storageelement candidate selection process is performed (Step S430). In thestorage element candidate selection process, storage elements that arecandidates for storing the target threshold value are selected. Becausea target threshold value can be stored in each of the empty storageelements, one of the empty storage elements is selected as a storageelement candidate. When the storage element candidate selection processhas been performed, the dot in the storage element candidate is turnedON (Step S440). In FIG. 18, the cross-hatching indicates the selectionof an empty storage element as the storage element candidate and theturning ON of the dot for that storage element candidate.

When the dot for the storage element candidate has been turned ON, thegroup selection process is performed (Step S450). In the group selectionprocess, a group Gq (where q is an integer from 1 to p) is selected fromamong p groups G1-Gp set in Step S410 (p is an integer equal to orgreater than 2, and here p=2).

When group Gq has been selected, evaluation value E1q indicating thedegree of dot dispersion is calculated on the basis of the dot formationpattern corresponding to the storage elements belonging to group Gq. Inother words, an evaluation value indicating the degree to which the dotsare dispersed evenly is calculated (Step S460). As is well known, adither mask can be generated with blue noise characteristics or greennoise characteristics as shown in FIG. 20 in order to form dots that aredispersed evenly. In this embodiment, the graininess index described inthe first embodiment is used as the evaluation value indicating thedegree to which the dots are dispersed evenly, in order to generate adither mask with these characteristics.

When the evaluation value E1q has been calculated, Step S450 and StepS460 are repeated until an evaluation value E1q has been calculated forall of the groups G1-Gp (here, G1-G2) (Step S470). When evaluationvalues E1q have been calculated for all groups G1-G2 (YES in Step S470),an evaluation value E1 is calculated on the basis of the calculatedevaluation values E11-E12 using Equation (3) (Step S480). In Equation(3), d-e are weighting factors. These weighting factors have beendetermined experimentally as constants so as to obtain good printquality. In other words, evaluation value E1 is a weighted overallevaluation value for the degree of dot dispersion in the dot formationpattern indicated by all of the decided storage elements in the dithermask, the dot formation pattern indicated by the decided storageelements corresponding to forward action, and the dot formation patternindicated by the decided storage elements corresponding to reverseaction.E1=d×E11+e×E12  (3)

When the evaluation value E1 has been calculated, Step S430 through StepS480 are repeated until evaluation values E1 have been calculated forall of the storage element candidates (empty storage elements) (StepS490). When evaluation values E1 have been calculated for all of thestorage element candidates (YES in Step S490), the first dither maskevaluation process has been completed. In this evaluation, thegraininess of the first pixel group and the graininess of the secondpixel group are the targets. However, the graininess of the combineddots from both the first and the second pixel groups can also be thetarget of evaluation.

When these evaluation values E1 are used, a first dither mask DM1 can begenerated having a dot formation pattern in which the dot placement isdispersed, whether the dots are formed during forward action or the dotsare formed during reverse action. Next, the second dither maskevaluation process is performed using this first dither mask DM1 as thestarting point. Because the second dither mask evaluation process issimilar to the method for generating the paired pixel control dithermask in the first embodiment (FIG. 12), the description of this processhas been omitted.

When halftone processing is performed using such a dither mask 62, thedispersion properties of forward action dots and reverse action dots canbe ensured even when there is a position shift between forward actiondots and reverse action dots. As a result, the dispersion properties ofthe dots in the entire image can be ensured, and deterioration in thegraininess of the image quality can be suppressed.

C. Third Embodiment

The third embodiment of the present invention will now be described.Because the third embodiment is realized using hardware similar to thatin the first and second embodiments, a description of the internalconfiguration of the printer 20 has been omitted. The difference withrespect to the first and second embodiment is that the error diffusionmethod is used in the halftone processing performed by the printer 20.In the third embodiment, as shown in FIG. 21, the upper right of theimage data is (0,0), the main scanning direction is x, the secondaryscanning direction is y, and the reference pixel is OB (x,y). These areused to successively determine whether or not a dot is to be formed. Theresulting density error (the difference between the gradation value tobe realized by the target pixel and the density actually realized by thepresence or absence of dot formation) is then diffused in thesurrounding pixels. As shown in FIG. 21, in the third embodiment,one-quarter of the density error is distributed to each of the fourpixels surrounding the reference pixel (x+1,y), (x−1,y+1), (x,y+1),(x+1,y+1). The error distribution rate can be different depending on thepixel error, and the pixel distribution range can be narrowed orexpanded. The error distribution range can also be switched depending onthe gradation value of the image.

In the third embodiment, in order to control the occurrence of paireddots, diffusion data for two paired dots Ped0(x,y), Ped1(x,y) iscalculated, and this is used to control the probability of paired dotsoccurring. This control is described using FIG. 22. In the thirdembodiment, as in the first embodiment, the pixels in which dots areformed during forward action of the print head 90 and the pixels inwhich dots are formed during reverse action of the print head arearranged in an alternating manner in the main scanning direction and thesecondary scanning direction as shown in FIG. 3C. Considered paireddots, as shown in FIG. 23, are pixels NL to the left of the referencepixel OB in the main scanning direction, and pixels NU up above thereference pixel in the secondary scanning direction.

In the paired pixel control error diffusion routine shown in FIG. 22,the reference pixel OB moves successively in the main scanning directionand the secondary scanning direction from the upper left (0,0) of theimage. First, the gradation data (x,y) of the reference pixel isinputted (Step S600). The gradation data (x,y) is data indicating thegradation value of the pixel positioned at (x,y) in the image.

Next, the arithmetic operation in Equation (4) is performed on thegradation data data (Step S610).dataX←data(x,y)+ed1(x,y)  (4)

In this arithmetic operation, the gradation data data is corrected usingdiffusion data ed1 (x,y) from processed pixels surrounding the referencepixel. The arithmetic operation performed on diffusion data is describedin detail below, and diffusion data ed1 (x,y) is the sum total of errordata diffused from the pixels determined to form dots towards thereference pixel OB (x,y).

In this embodiment, the arithmetic operation shown in Equation (5) isperformed on the corrected data dataX (Step S620).dataX2←dataX+w0×Ped0+w1×Ped1  (5)

Here, Ped0, Ped1 is paired pixel diffusion data from the processedpixels. Paired pixel diffusion data Ped0, Ped1 determines whether or notthe probability of paired dots being formed in both the reference pixelOB and the adjacent pixels NL, NU shown in FIG. 23 is deficient withrespect to the prediction value for the probability of paired dotsoccurring. This point will be described in greater detail.

Because the range of gradation data is expressed by 0-255 when thegradation value of a reference pixel OB is expressed by 8-bit gradationdata data, the probability PK (referred to as the incidence below) thata dot will be formed in both the reference pixel and an adjacent pixelis PK=(data/255)². For the sake of arithmetic operational efficiency,the incidence PK is treated as PK=(data)²/255 inside the printer 20. Ina case in which the gradation values in the image are uniform, thepaired dot incidence in the main scanning direction and the paired dotincidence in the secondary scanning direction are treated as the samevalue. However, in reality, the gradation data data for the referencepixel and each adjacent pixel could be different, so these aredistinguished from each other and treated as PK0(x,y), PK1(x,y).

Concerning the paired dot incidence, when it has been determined(described below) whether or not a dot has been formed with respect tothe reference pixel OB, it is also determined whether or not paired dotshave been formed. The determination results are used to diffuse theshift from the prediction value for the paired dot incidence to thesurrounding pixels. This diffusion is described in detail below.However, the error diffusion portion Δpk0(x,y), Δpk1(x,y) of theprediction values for the incidence from the surrounding pixels is usedto determine the paired pixel diffusion data Ped0, Ped1 from Equation(6) below.Ped0←PK0(x,y)+Δpk0(x,y)Ped1←PK1(x,y)+Δpk1(x,y)  (6)

The arithmetic operation in Equation (5) is performed using the pairedpixel diffusion data Ped0, Ped1 determined in Equation (6) to determinethe corrected gradation data dataX2. In Equation (4), w0, w1 areweighting factors for adjusting the paired dot incidence. The valueshould be 1 in a case in which the paired dot incidence approaches theincidence in a white noise mask. In a case in which the graininess ofthe image is the emphasis, the weighted factors should be less than 1.

When the corrected gradation data dataX2 has been determined, the sizesof the corrected gradation data dataX2 and the threshold value Thr arecompared (Step S630). The threshold value Thr used in the comparison canbe a fixed value (for example, 127) or a value that depends on thegradation data data. When the threshold value Thr is a value thatdepends on the gradation data data, phenomena such as tailing can beeliminated.

When, as a result of the determination in Step S630, the correctedgradation data dataX2 is greater than the threshold value Thr, a dot isformed. The value 1 is assigned to the dot data Ddata (x,y) (Step S640),and an arithmetic operation is performed on the result indicating a caseof dot formation (Step S645). When, as a result of the determination inStep S630, the corrected gradation data dataX2 is less than thethreshold value Thr, a dot is not formed. The value 0 is assigned to thedot data Ddata (x,y) (Step S650), and an arithmetic operation isperformed on the result indicating a case of no dot formation (StepS655). The value of the dot data Ddata(x,y) is referenced during theinterlacing explained in the first embodiment, and dot formation isperformed in accordance with this data.

The following is a description of the arithmetic operations performed onthe result values in Step S645 and Step S655. In a case in which a dotis formed in the reference pixel OB (Step S640) and in a case in which adot is not formed in the reference pixel (Step S650), the densityrealized in the reference pixel OB has to be determined and prepared forthe subsequent error diffusion process. This is a result value res(x,y)related to the gradation value. In this embodiment, the paired dotincidence is controlled. Similarly, whether or not paired dots occurwith respect to the reference pixel OB is determined as result valueresP0, resP1.

More specifically, the result value res(x,y) for the gradation value is255 in a case in which a dot is formed in the reference pixel OB (StepS640), and 0 in a case in which a dot is formed in the reference pixel(Step S650). The result value does not have to be set to 255 or 0. Forexample, in a case in which a low concentration ink and/orsmall-diameter ink droplet is used, the result value when a dot isformed can be 96. In other words, the concentration of the ink and thesize of the ink droplet can be combined in setting the value when a dotis formed. When the color of the printing paper P is a color other thanwhite, the result value when a dot is not formed can be a predeterminedvalue greater than zero. Preferably, the setting is used to adjust thedensity of the image that is actually printed.

In this embodiment, the result value for paired dots resP0, resP1 isdetermined along with the result value res(x,y) for the gradation value(Step S645, Step S655). This is used to bring the paired dot incidencecloser to the prediction value. More specifically, in a case in which ithas been determined that a dot is to be formed in the reference pixel OB(Step S640), as shown in FIG. 24, it is determined whether or not a dotis formed in the adjacent pixel NL(x−1,y) to the left of the referencepixel OB(x,y) (Step S660). When a dot is formed, paired dots occur andthe value 255 is set in resP0 (Step S661). When a dot is not formed inthe adjacent pixel NL, paired dots do not occur and the value 0 is setin resP0 (Step S662).

Whatever the determination result for adjacent pixel NL, it isdetermined whether or not a dot is formed in the adjacent pixelNU(x,y−1) above the reference pixel OB(x,y) (Step S665). When a dot isformed, paired dots occur and the value 255 is set in resP1 (Step S666).When a dot is not formed in the adjacent pixel NU, paired dots do notoccur and the value 0 is set in resP1 (Step S667).

In a case in which a dot is not formed in Step S650, a paired dot doesnot occur with either the adjacent pixel NL to the left of the referencepixel OB(x,y) or the adjacent pixel NU above the reference pixel, so thevalue 0 is set in both result values resP0, resP1 (Step S655).

The processing described above is performed to determine dot formationin the reference pixel OB, and complete processing of paired pixeldiffusion data for error diffusion and paired pixel control. Next, theerror diffusion processing (Step S670) and the paired pixel errordiffusion processing (Step S680) are executed in successive order.Because error diffusion processing is well known, the description hasbeen simplified. In this process, the difference between the resultvalue res(x,y) set in Step S645 or Step S655 and the gradation dataX forthe reference pixel OB corrected using error diffusion (arithmeticoperation in the first equation) is determined, and this is distributedto the four surrounding pixels shown in FIG. 21. More specifically, thedensity error ed and the error diffusion buffer er are determined usingthe following equations.ed=dataX−res(x,y)er(x+1,y)=er(x+1,y)+ed/4er(x−1,y+1)=er(x−1,y+1)+ed/4er(x,y+1)=er(x,y+1)+ed/4er(x+1,y+1)=er(x+1,y+1)+ed/4

Therefore, the diffusion data ed1(x,y) described in Step S610 in FIG. 22corresponds to the sum total for each pixel of the error diffused toeach error diffusion buffer er in the arithmetic operation from eachpixel surrounding the reference pixel.

Similarly, in the paired pixel error diffusion process, the diffusiondata Ped0(x,y) for the adjacent pixel NL in the main scanning directionand the diffusion data Ped1(x,y) for the adjacent pixel NU in thesecondary scanning direction are determined, and the incidence errorsPerr0, Perr1 and the paired pixel error diffusion buffers erP0, erP1 foreach are determined using the following equations.

Related to Adjacent Pixel NL:Perr0=Ped0−resP0erP0(x+1,y)=erP0(x+1,y)+Perr0/4erP0(x−1,y+1)=erP0(x−1,y+1)+Perr0/4erP0(x,y+1)=erP0(x,y+1)+Perr0/4erP0(x+1,y+1)=erP0(x+1,y+1)+Perr0/4Related to Adjacent Pixel NU:Perr1=Ped1−resP1erP1(x+1,y)=erP1(x+1,y)+Perr1/4erP1(x−1,y+1)=erP1(x−1,y+1)+Perr1/4erP1(x,y+1)=erP1(x,y+1)+Perr1/4erP1(x+1,y+1)=erP1(x+1,y+1)+Perr1/4

Therefore, the error diffusion portions Δpk0(x,y), Δpk1(x,y) describedusing Equation (3) in Step S620 of FIG. 22 correspond to the sum totalfor each pixel of the error with the prediction value related to thepaired dot incidence diffused to each paired pixel error diffusionbuffer erP0, erP1 using the arithmetic operation from each pixelsurrounding the reference pixel.

After this processing has been performed, it is determined whether theprocessing of all of the pixels constituting the image has beencompleted (Step S690). When determination for all of the pixels has notbeen completed, the process returns to Step S600, the reference pixel isadvanced one increment, and the processing is repeated from the input ofgradation data data(x,y) for the reference pixel. When determination forall of the pixels has been completed, the process is exited at END, andthe processing routine is completed.

In the third embodiment described above, the paired dot incidence isbrought closer to the prediction value while error diffusion isperformed. In this embodiment, the prediction value is the square of thedot incidence. As in the first embodiment, the weighting factors can beadjusted w0, w1 to obtain a value of approximately 0.8, which is a whitenoise characteristic. Because the algorithm in this embodiment basicallyemploys error diffusion, the dot distribution properties are close toblue noise characteristics and very good. In addition, because thepaired dot incidence is controlled so as to be at the same level aswhite noise characteristics, the coverage and paired dot incidence donot change very much even when a shift occurs in the dot formationposition of dots during forward action and reverse action of the printhead 90. As a result, image quality deterioration can be sufficientlysuppressed even when there is a shift in the formation position of dotsin bi-direction printing.

Also, in this embodiment, the processing related to paired pixels isadded to the error diffusion algorithm. As a result, the processingroutine can be realized via slight corrections and additions. Becausecontrol of the error diffusion range for gradation data and the paireddot incidence can be realized using variables, the error diffusionalgorithm does not have to be changed even in a case in which thebi-directional printing mode and/or the paired pixel range has beenchanged as shown, for example in FIG. 8 and FIG. 15. Alternately, theerror diffusion range is easy to change without altering paired pixelcontrol.

D. Fourth Embodiment D-1. Device Configuration

The following is a description of the fourth embodiment of the presentinvention. FIG. 25 is a schematic block diagram of the printer 20 in thefourth embodiment. The hardware of the printer 20 in the fourthembodiment is the same as that of the first embodiment (see FIG. 1)except that the contents of the dither mask stored in the EEPROM 60 aredifferent. Description of the internal configuration of the printer 20that is the same as that of the first embodiment has been omitted. Inthe fourth embodiment, the printing process performed by the printer 20differs from that of the first embodiment (see FIG. 4).

Two dither masks are stored in the EEPROM 60 of the fourth embodiment.The size of the first and second dither masks 61, 62 used in thisembodiment is 64×64, and threshold values from 0 to 256 are stored in4096 storage elements. Each threshold value is used in the halftoneprocessing described below. In the first and second dither masks 61, 62,the placement of each threshold value is decided so that thecharacteristics basically emphasize the dispersion properties of thedots. The characteristics of the first and second dither mask 61, 62used in this embodiment are described below, but the first dither mask61 is a so-called blue noise mask, and the second dither mask 62 is adither mask for suppressing any deterioration in image quality andrealizing high image quality in bi-directional printing.

D-2. Printing Process

The printing process performed by the printer 20 in this embodiment willnow be described. FIG. 26 is a flowchart of the printing processperformed by the printer 20 in this embodiment. The step numbers in theflowchart used to explain this embodiment are the step numbers of theflowchart in FIG. 26. Here, the user operates the control panel 99 toinitiate the printing process by performing a printing instructionoperation for a predetermined image stored in the memory card MC. Whenthe printing process has been initiated, the CPU 40 first reads andinputs the RGB-formatted image data ORG to be printed from the memorycard MC via the memory card slot 98 (Step S110).

When the image data ORG has been inputted, the setting for the paper tobe printed is received (Step S115). Here, the user sets the paper sizeusing the control panel 99. A particular paper size (such as A4) is setas the default setting, and the default paper size is used unlessotherwise indicated by the user. The paper size setting is stored in thePageSize variable, and this is referenced in the subsequent processing.After the paper size has been set, the CPU 40 references the look-uptable (not shown) stored in the EEPROM 60, and performs color conversionon the image data ORG from the RGB format to the CMYKLcLm format (StepS120).

When the color conversion process has been completed, the paper size isdetermined (Step S125). The paper size is determined by referencing thePageSize variable saved in advance. As a result of the paper sizedetermination, the CPU 40 performs the first halftone process when thepaper size is less than A4 such as in the case of photographic paper(cabinet size, L size, 2L size, and the like) (Step S130), and performsthe second halftone process when the paper size is A4 or greater (StepS135). Whether the first or second halftone process is performed, theCPU 40 performs the processing of the dot data generating unit 42 andperforms halftone processing in which the image data is converted toON/OFF data for the dots of each color. Only the content of the processis different. In this embodiment, the first and second halftoneprocesses are performed using the dither method. The first and seconddither mask 61, 62 used in this process are repeatedly applied in themain scanning direction and the secondary scanning direction dependingon whether the inputted data is aligned in the main scanning directionor the secondary scanning direction. The first and second halftoneprocessing in this embodiment is controlled so that the generated dotdata has predetermined characteristics. The content of the controldepends on the characteristics of the first and second dither masks 61,62. The first and second halftone processes are not limited to binaryON/OFF dot processing. It can also be multi-value processing such asON/OFF processing of large dots and small dots. Also, the image dataprovided to the halftone process can be obtained from image processingsuch as resolution conversion processing and smoothing processing.

When either the first or second halftone process is performed, the CPU40 performs overlapping and interlacing alternatingly aligned withrespect to dot data to be printed in a single main scanning unit,harmonized with the nozzle arrangement of the printer 20, the paper feedrate, and other parameters (Step S140). When the overlapping andinterlacing have been performed, the CPU 40 performs the processing of aprinting unit 43, driving the print head 90, carriage motor 70, andmotor 74, and executing the printing (Step S150).

When the printing process is executed (FIG. 26), dot formation isperformed using either the first dither mask 61 or the second dithermask 62 depending on the paper size. The printer 20 forms dots byejecting ink from the print head on a plurality of different timings (inother words, forward action and reverse action) in the common printregion of the print medium while changing the ink ejection position withrespect to the print medium, and a printed image is outputted in whichthe dots formed during the forward action and the dots formed during thereverse action are aligned with each other.

D-3. Halftone Processing

The following is a description of the characteristics of the halftoneprocess in this embodiment. In this embodiment, the first or secondhalftone process is performed depending on the set size of the paper(Step S130 or Step S135). The first halftone process shown as Step S130in FIG. 26 will be described first. In the first halftone process,whether or not to form a dot in a pixel position is decided by comparingthe gradation values of a pixel belonging to a first pixel groupbelonging to a dot to be formed during forward action of the print head90 and a pixel belonging to a second pixel group belonging to a dot tobe formed during reverse action of the print head 90 to the first dithermask 61 stored in the EEPROM 60. The first dither mask 61 used in thefirst halftone process is a so-called blue noise mask, and a dot ON/OFFdecision is made by comparing the gradation value of image data to thethreshold value for the corresponding position in the first dither mask61.

A blue noise mask is a dither mask in which the threshold values arearranged so that the arrangement of dots to be generated has blue noisecharacteristics. In blue noise characteristics, the distribution of dotsformed has a noise characteristic possessing a peak in the spatialfrequency region on the higher-frequency side relative to thelow-frequency region at or below a predetermined spatial frequency.These characteristics give strong priority to the dispersion propertiesof the dots. Dot data is generated in the first halftone process usingthis blue noise mask. As a result, when the paper size is less than A4,for example, a photo size (L size), the dispersion properties of thedots is given priority, and dot data is generated to performhigh-quality printing with superior graininess. In the first halftoneprocess, no distinction is made between the first and second pixelgroups when compared to the threshold values in the dither mask.

The second halftone process will be described next. In the secondhalftone process (Step S135), dot data is also generated via acomparison to a threshold value in the dither mask. However, a seconddither mask 62 is used in the second halftone process. The second dithermask 62 in this embodiment has the same content as the dither mask 62 inthe first embodiment (see FIG. 1), and the generation method is the sameas the generating method used by the dither mask in the first embodiment(FIG. 12). Therefore, further description has been omitted.

D-4. Effect of the Fourth Embodiment

In the printer 20 of the fourth embodiment having the configurationdescribed above, image data ORG is received, and the control unit 30performs the processing shown in FIG. 26 to print an image on printingpaper P. In a case in which the size of the printing paper P is lessthan A4, for example, photograph size (L-size, or the like), halftoneprocessing is performed and dot data generated using the first dithermask 61 which has blue noise characteristics. In a case in which thesize of the printing paper P is A4 or greater, halftone processing isperformed and dot data generated using the second dither mask 62 inwhich the probability of dots being formed in paired pixels is high. Inboth cases, the data is converted to the final dot distribution. As aresult, in a case in which the paper size is small, an image can beformed with an especially sparse dot distribution and superiorgraininess in the low gradation region. In a case in which the papersize is large, the second dither mask 62 is used to increase the qualityof an A4-size image to the human eye, which is usually viewed from agreater distance than a photograph-sized image. Here, the change in thepercentage of paired dots formed in the 0-127 gradation region hardlychanges, and density unevenness is less likely to occur.

E. Fifth Embodiment

The following is an description of the fifth embodiment of the presentinvention. The hardware of the printer 20 in the fifth embodiment is thesame as that of the fourth embodiment (see FIG. 25). In the fifthembodiment, the printing process performed by the printer 20 differsfrom that of the fourth embodiment (see FIG. 26).

E-1. Printing Process

The printing process performed by the printer 20 in the fifth embodimentwill now be described. FIG. 27 and FIG. 28 are flowcharts of theprinting process performed by the printer 20 in this embodiment. Thestep numbers in the flowchart used to explain this embodiment are thestep numbers of the flowcharts in FIG. 27 and FIG. 28. Here, the useroperates the control panel 99 to initiate the printing process byperforming a printing instruction operation for a predetermined imagestored in the memory card MC. When the printing process has beeninitiated, the CPU 40 first reads and inputs the RGB-formatted imagedata ORG to be printed from the memory card MC via the memory card slot98 (Step S110).

When the image data ORG has been inputted, the setting for the paper tobe printed is received (Step S115). Here, the user sets the paper sizeusing a paper setting dialog box PDB displayed on the control panel 99.A particular paper size (such as A4) is set as the default setting, andthe default paper size is used unless otherwise indicated by the user.The paper size setting is stored in the PageSize variable, and this isreferenced in the subsequent processing. After the paper size has beenset, the CPU 40 references the look-up table (not shown) stored in theEEPROM 60, and performs color conversion on the image data ORG from theRGB format to the CMYKLcLm format (Step S120).

When the color conversion process has been completed, the paper side isdetermined (Step S125). The paper size is determined by referencing aPageSize variable saved in advance. As a result of the paper sizedetermination, the CPU 40 sets the paper quality parameter DF to 1 whenthe paper size is less than A4 such as in the case of photographic paper(cabinet size, L size, 2L size, and the like) (Step S130). When thepaper size is A4 or greater, the value 2 is set in the image qualityparameters DF (Step S135). After the dot data generating process hasbeen performed (Step S140), the CPU 40 performs overlapping andinterlacing alternatingly aligned with respect to dot data to be printedin a single main scanning unit, harmonized with the nozzle arrangementof the printer 20, the paper feed rate, and other parameters (StepS150). When the overlapping and interlacing have been performed, the CPU40 performs the processing of a printing unit 43, driving the print head90, carriage motor 70, and motor 74, and executing the printing (StepS160). The printing process is then completed.

The dot data generating process described above (Step S140) is theso-called halftone process in which data for turning dots ON/OFF forprinting (forming or not forming dots) is generated from image dataafter color conversion. This process is described using FIG. 28. Whenthe dot data generating process has been started (Step S140), the CPU 40first references the image quality parameter DF (Step S141). When theimage quality parameter DF is 1, the first halftone process is performed(Step S143). When the image quality parameter DF is 2, the secondhalftone process is performed (Step S145). Whether the first or secondhalftone process is performed, the CPU 40 performs the processing of thedot data generating unit 42 and performs halftone processing in whichthe image data is converted to dot data of each color. Only the contentof the process is different. In this embodiment, the first and secondhalftone process are performed using the dither method. The first andsecond dither mask 61, 62 used in this process are repeatedly applied inthe main scanning direction and the secondary scanning directiondepending on whether the inputted data is aligned in the main scanningdirection or the secondary scanning direction. The first and secondhalftone processing in this embodiment is controlled so that thegenerated dot data has predetermined characteristics. The content of thecontrol depends on the characteristics of the first and second dithermasks 61, 62. The first and second halftone process are not limited tobinary ON/OFF dot processing. It can also be multi-value processing suchas ON/OFF processing of large dots and small dots. Also, the image dataprovided to the halftone process can be obtained from image processingsuch as resolution conversion processing and smoothing processing.

When the printing process is executed (FIG. 27, FIG. 28), dot formationis performed using either the first dither mask 61 or the second dithermask 62 depending on the paper size. The printer 20 forms dots byejecting ink from the print head on a plurality of different timings (inother words, forward action and reverse action) in the common printregion of the print medium while changing the ink ejection position withrespect to the print medium, and a printed image is outputted in whichthe dots formed during the forward action and the dots formed during thereverse action are aligned with each other.

E-2. Halftone Processing

The following is a description of the characteristics of the halftoneprocess in the fifth embodiment. In this embodiment, the first or secondhalftone process is performed depending on the set size of the paper(Step S143 or Step S145). The first halftone process shown as Step S143in FIG. 28 will be described first. In the first halftone process,whether or not to form a dot in a pixel position is decided by comparingthe gradation values of a pixel belonging to a first pixel group and apixel belonging to a second pixel group to the first dither mask 61stored in the EEPROM 60. The first dither mask 61 used in the firsthalftone process is a so-called blue noise mask, and a dot ON/OFFdecision is made by comparing the gradation value of image data to thethreshold value for the corresponding position in the first dither mask61.

The second halftone process will be described next. In the secondhalftone process (Step S145), dot data is also generated via acomparison to a threshold value in the dither mask. However, a seconddither mask 62 is used in the second halftone process. The second dithermask 62 in this embodiment has the same content as the dither mask 62 inthe first embodiment (see FIG. 1), and the generation method is the sameas the generating method used by the dither mask in the first embodiment(FIG. 12). Therefore, further description has been omitted.

E-3. Effect of the Fifth Embodiment

In the printer 20 of the fifth embodiment having the configurationdescribed above, image data ORG is received, and the control unit 30performs the processing shown in FIG. 27 and FIG. 28 to print an imageon printing paper P. In a case in which the size of the printing paper Pis A4 or less, for example, photograph size (L-size, or the like),halftone processing is performed and dot data generated using the firstdither mask 61 which has blue noise characteristics. In a case in whichthe size of the printing paper P is A4 or greater, halftone processingis performed and dot data generated using the second dither mask 62 inwhich the probability of dots being formed in paired pixels is high. Inboth cases, the data is converted to the final dot distribution. As aresult, in a case in which the paper size is small, an image can beformed with an especially sparse dot distribution and superiorgraininess in the low gradation region. In a case in which the papersize is large, the second dither mask 62 is used to increase the qualityof an A4-size image to the human eye, which is usually viewed from agreater distance than a photograph-sized image. Here, the change in thepercentage of paired dots formed in the 0-127 gradation region hardlychanges, and density unevenness is less likely to occur.

E-4. Modification Examples of Fourth and Fifth Embodiments

In this embodiment, the printer 20 shown in FIG. 25 performs theprinting process from the input of image data to the printing itself.However, a printer 20 can be connected to a computer PC, and theprocessing from Step S110 to Step S140 in FIG. 26 and the processingfrom Step S110 to Step S150 shown in FIG. 27 and FIG. 28 can beperformed by the computer PC. In this case, the paper size determination(Step S125 in both FIG. 26 and FIG. 27) can be performed by checking amember number managed by a printer driver.

Because printing can be performed smoothly between a computer PC and aprinter using a general operating system such as Windows™, theinformation needed for printing can be managed as a collection ofparameters called “members”. The following are some of the members usedin Windows™.

dmOrientation (paper orientation)

dmPaperSize (paper size)

dmPaperLength (paper length)

dmPaperWidth (paper width)

dmPosition (paper position)

dmScale (scaling)

dmCopies (number of printed copies)

dmDefaultSource (default paper tray)

dmPrintQuality (printing resolution)

dmColor (color or monochrome printing in color printer)

dmDuplex (enable two-sided printing)

Because these members are set in a process such as print setup, a printdriver or the like can determine the paper size setting by referencingthese members.

In a case in which the first and second halftone processing in theprinting process shown in FIG. 26, FIG. 27, and FIG. 28 are performed bya computer PC, when the determination in Step S125 is made, dmPaperSize(paper size) is called up from the members, the paper size is determinedfrom the value, and either the first or second halftone process isexecuted accordingly. While the viewing distance for printed materialdepends largely on paper size, it can also depend on the size of theregion in which the image is actually printed. Therefore, in thedetermination made in Step S125, the following elements can also beconsidered instead of the paper size alone. The determination can bemade using any one of these elements alone or using any combination ofelements.

(A) Paper Size

(B) Paper Orientation (Landscape or Portrait)

(C) Paper Margins (Margins)

(D) Printing Resolution (Resolution Widthwise, Resolution Lengthwise)

(E) Aspect of Region For Printed Image

(F) Aspect Ratio of Image to be Printed (Aspect)

(G) Paper Type (Plain Paper, Fine Paper, Photo Paper, Overhead ProjectorSheet, and the like)

Based on these elements used alone or in combination, the first dithermask 61 is used when the printing region is small, and the second dithermask 62 is used when the printing region is large. Also, the firstdither mask 61 can be used when the resolution is low, and the seconddither mask 62 can be used when the resolution is high. This is because,when the resolution is high, the printing process takes more time, inkis absorbed by the print medium (such as paper) during printing usingink, the print medium stretches, cockling occurs, and a shift in thelanding position of ink droplets is more likely to occur.

The type of dither mask to be used or a reference pointer for the dithermask can also be prepared as one of the members. When the paper sizemember is set, the member indicating the type of dither mask or themember indicating the reference pointer for the dither mask can bechanged directly in accordance with this value.

F. Sixth Embodiment F-1. Dither Mask

The following is a description of the sixth embodiment of the presentinvention. The hardware of the printer 20 in the sixth embodiment is thesame as that in the fourth and fifth embodiments (see FIG. 25). Also,the printing process performed by the printer 20 is the same as that ofthe fourth and fifth embodiments (FIG. 26, FIG. 27, FIG. 28) except forthe first dither mask 61 used in the first halftone process. In thesixth embodiment, the first and second halftone processing is performedusing the so-called dither method.

The following is a comparison of the characteristics of the first dithermask 61 in this embodiment and the first dither mask 61 in the fourthand fifth embodiments.

(1) Similarities:

The size of the dither mask is 64×64 in both.

A blue noise mask giving priority to dispersion properties is used.

(2) Differences:

In the fourth and fifth embodiments, the threshold values for the 64×64dither mask were prepared as a simple blue noise mask. By contrast, thedither mask in the sixth embodiment is created to take into account thedispersion properties of dots in the first pixel group to which the dotsformed during forward action of the print head 90 belong and the dots inthe second pixel group to which the dots formed during reverse action ofthe print head 90 belong.

These differences will now be described. An image is formed by theprinter 20 from dots formed during forward action of the print head 90and dots formed during reverse action of the print head. Therefore, thedistribution of dots in an image obtained using certain gradation valuesis a distribution of dots formed during both the forward action andreverse action of the print head 90. Conventionally, the thresholdvalues for the dither mask are decided while targeting improveddispersion properties for the dots in this case. By contrast, the dithermask used in the sixth embodiment takes into account the dispersionproperties of dots in the first pixel group to which the dots formedduring forward action of the print head 90 belong and the dots in thesecond pixel group to which the dots formed during reverse action of theprint head 90 belong. In other words, the dither mask used in the sixthembodiment, as shown in FIG. 16, takes into account the dispersionproperties of the dots themselves formed during forward action (FIG.16A) and the dispersion properties of the dots themselves formed duringreverse action (FIG. 16B) when a particular image is formed. A methodfor generating such a dither mask is described below, but any methodable to generate a dither mask with these characteristics can be used.

A printer 20 of the sixth embodiment using a dither mask with thesecharacteristics is able to sufficiently suppress deterioration in imagequality even when a shift occurs in the position of dots formed duringthe forward action and reverse action of the print head 90. This isbecause a dither mask is used which takes into account the dispersionproperties of the first pixel group to which dots formed during forwardaction of the print head 90 belong and the second pixel group to whichdots formed during reverse action of the print head 90 belong. In orderto ensure the dispersion properties of the dots belonging to both groupseven when a shift occurs in the formation position of dots duringforward action and reverse action, a slight decrease in the dispersionproperties of the dots remains in a case in which the dots of bothgroups overlap in the common region. This is because the graininess in acase in which the dots belonging to the two pixel groups are combinedindicates a strong correlation with the graininess of the individualdots belonging to each pixel group.

F-2. Dither Mask Generating Method

The steps in the generation method for a first dither mask 61 with thesecharacteristics are shown in FIG. 29. The step numbers in the flowchartused to explain this embodiment are the step numbers of the flowchart inFIG. 29. As shown in the figure, when generating the first dither mask61, threshold values are first prepared based on the size of the firstdither mask 61 (Step S310). In this embodiment, the size of the firstdither mask 61 is 64×64. However, the description has been simplified byusing an 8×8 sized mask with 64 storage elements. In Step S310,threshold values 0-63 are prepared. In other words, a threshold value isprepared for each storage element.

When the threshold values have been prepared, the target threshold valueselection process is performed (Step S320). In the target thresholdvalue selection process, a threshold value is selected as a targetthreshold value among the prepared threshold values 0-63 not yet storedin a storage element. In this embodiment, the target threshold valuesare selected from among the prepared threshold values in successiveorder from the smallest to the largest. As shown in FIG. 18, in a casein which threshold values 0-3 have already been stored in storageelements constituting the dither mask by performing the steps describedbelow, the target threshold value selected in the next Step S320 isvalue 4.

When the target threshold values have been selected, the first dithermask evaluation process is performed (Step S400). In the first dithermask evaluation process, in a case in which a target threshold value isto be stored in a storage element that is not yet storing a preparedthreshold value, an evaluation value E1 is calculated for each emptystorage element which indicates the extent of dot dispersion in a dotformation pattern indicating the arrangement of storage elements inwhich threshold values have already been stored. The calculation methodfor this evaluation value E1 is described below. However, in thisembodiment, a smaller evaluation value E1 indicates better dotdispersion properties and is good from the standpoint of the graininessof the printed image.

Next, the stored element is decided using the evaluation value E1 (StepS330), and it is determined whether or not the deciding process has beencompleted for all of the storage elements (Step S340). When the processhas been performed for all of the storage elements, the process returnsto Step S320, and the process described above is repeated. When thefirst dither mask evaluation process described above has been performedand threshold values have been stored in all of the storage elements(YES in Step S340), the evaluation of the first dither mask 61 hasended, and the dither mask generating process is complete. The firstdither mask evaluation process in this embodiment is similar to thefirst dither mask evaluation process performed in the second embodiment(FIG. 19), so further description has been omitted.

When halftone processing is performed using such a first dither mask 61,the dispersion properties of forward action dots and reverse action dotscan be ensured even when there is a position shift between forwardaction dots and reverse action dots. As a result, the dispersionproperties of the dots in the entire image can be ensured, anddeterioration in the graininess of the image quality can be suppressed.

The present invention was described above with reference to embodiments,but the invention is by no means limited to these embodiments. Thepresent invention can be realized in a variety of ways in a range thatdoes not change the essentials of the invention. For example, thepresent invention can be embodied in a printer for monochromaticprinting or in a line printer in which print heads are provided in thewidth direction of the paper. Also, the processing shown in FIG. 4, FIG.22, FIG. 26, FIG. 27, and FIG. 28 can be embodied by a computer (orserver connected to a network) rather than by a printer. In addition,the processing can be realized by hardware (for example, an RIP providedbetween printers).

In the embodiments, the description was limited to a first and a secondpixel group, which are a group of pixels in which dots are formed duringforward action, and a group of pixels in which dots are formed duringreverse action in bi-directional printing. However, the pixel groups canbe established in various ways as long as the printing conditions aredifferent. For example, dots formed during multiple passes of aso-called multi-pass printer in which a raster is created by multiplemain scans can be divided into different pixel groups, and the adjacentpixels can be set between each pixel group to control the paired dotincidence. Alternatively, the pixels in which dots are formed by nozzlecolumns ejecting ink can be divided into groups for each nozzle column.

In addition, the percentage of dots formed during forward action andduring reverse action can be changed depending on the configuration usedto control the probability of paired dots being formed, or theconfiguration used to eject ink droplets of different sizes from nozzlesto form dots. In a case in which large, medium and small dots areseparated or in a case in which dots of different concentrations areseparated, the probability of paired dots being generated is preferablycontrolled using one type of dot from the smallest diameter dot(lightest ink) or using multiple types of dots from the smallestdiameter dot (lightest ink). In other words, because those among thevarious types of dots that are large-diameter dots (the darkest dots)are formed in the high gradation region, the probability of these dotsbeing paired dots need not be controlled.

In the fourth through sixth embodiments, there was switching between twodither masks. However, three or more dither masks can be prepared, andswitching can be performed between more than two dither masks dependingon the size of the printing region or some other factor. Also, the firsthalftone process can be performed using another method as long as theprocess is the same as when a dither mask is used. For example, thehalftone process described above using a dither method and the firstdither mask can also be realized using the error diffusion method. Inprocessing using the error diffusion method, the arrangement of dots isclose to blue noise characteristics. Therefore, halftone processing canbe performed using the error diffusion method when the paper size issmaller than a predetermined value or the print region is smaller than apredetermined value, and halftone processing can be performed using adither mask in which the probability of paired dots being generated isclose to k² when the paper size is greater than a predetermined value orthe print region is greater than a predetermined value. This has thesame effects as the fourth through sixth embodiments. The secondhalftone processing can also be realized using the error diffusionmethod. In this case, a buffer is provided for diffusing the densityerrors generated by the reference pixel, a counter is provided to countthe number of dots formed in adjacent pixels, the excess or deficiencyis calculated with respect to the prediction value for paired dots, andthis is used to correct the gradation values in the surrounding pixels.

The printing device, printing method, and dither mask generating methodof the present invention were described above as embodiments. A printingdevice in the first and second embodiments controls the number of paireddots using a dither mask to which certain characteristics have beenimparted. A printing device in the fourth through sixth embodimentscontrols the number of paired dots using a dither mask to which certaincharacteristics have been imparted when the paper size is A4 or greater.Therefore, one can determine whether the invention in the presentapplication has been embodied even when the characteristics of thedither mask have not been analyzed. In other words, in a case in which adither mask having high dispersion properties is used as shown in FIG.9, the probability K of paired dots is close to zero in the region ofthe image with low gradation values (for example, 0-50/255, dotincidence: 0-0.2, and this is very far from k². Therefore, it can bedetermined whether the invention in the present application has beenembodied in the first and second embodiments by the use of thegraininess index shown in Equations (1) and (2), and in the fourththrough sixth embodiments by the use of different halftone processingdepending on the paper size and by the use of the graininess index shownin Equations (1) and (2), where the dispersion properties in the imageare above a predetermined value, and the probability K of paired dotsbeing formed is, for example, 0.2·k²≦K≦0.8·k² relative to the dotincidence k. In a case in which the error diffusion method is used, thenoise characteristics are similar to blue noise characteristics.Therefore, it is also easy to determine whether the invention in thepresent application has been embodied by measuring whether the paireddot incidence is within a predetermined gradation range for the image.

What is claimed is:
 1. A printing device for forming dots in pixels arranged on a printing medium according to an input image data and printing an image to perform printing of an output printed image, the printing device comprising: a printing unit configured to print the output printed image by dividing formation of the dots in a common region into a plurality of pixel groups, in a case where first and second pixels belonging, respectively, to two pixel groups among the plurality of pixel groups are proximal pixels in a predetermined gradation range in which probabilities k1 and k2 at which a dot is formed in the first and second pixels are such that k1<0.5 and k2<0.5, a probability K of a dot being formed on both of the proximal pixels is set to be close to k1·k2.
 2. The printing device of claim 1, wherein the probability K of a dot being formed in both proximal pixels is set to be closer to k1·k2 in a case in which the size of a printing region in the printing medium is equal to or greater than a first predetermined value than in a case in which the size of a printing region in the printing medium is less than the first predetermined value or a second predetermined value that is smaller than the approximate predetermined value.
 3. The printing device of claim 1, wherein whether or not to form a dot is decided by comparing the gradation value of the pixel to a threshold value of a dither mask prepared in advance.
 4. The printing device in claim 1, wherein the printing unit performs a reciprocating action with respect to a main scanning direction, and prints the output printed image during both main scanning in the forward action and main scanning in the reverse action, and the first pixel group to which the first pixel belongs is a group of pixels in which dots are formed by main scanning in the forward action, and the second pixel group to which the second pixel belongs is a group of pixels in which dots are formed by main scanning in the reverse action.
 5. The printing device of claim 4, wherein the dots formed by main scanning in the forward action and the dots formed by main scanning in the reverse action are arranged in an alternating manner in both the main scanning direction and a secondary scanning direction intersecting the main scanning direction; and the proximal pixels are a combination of one pixel and another pixel adjacent to the pixel in the main scanning direction, and a combination of a pixel and another pixel adjacent to the pixel in the secondary scanning direction.
 6. The printing device of claim 4, wherein the dots formed by main scanning in the forward action and the dots formed by main scanning in the reverse action are arranged in an alternating manner in the main scanning direction, and are arranged so that the dots formed by main scanning in the forward action or the dots formed by main scanning in the reverse action are contiguous in a secondary scanning direction intersecting the main scanning direction; and the proximal pixels are a combination of one pixel and another pixel adjacent to one side of the one pixel in the main scanning direction, and a combination of the one pixel and pixels adjacent to the adjacent pixel on either side in the secondary scanning direction.
 7. The printing device of claim 4, wherein the dots formed by main scanning in the forward action and the dots formed by main scanning in the reverse action are arranged in an alternating manner in a secondary scanning direction intersecting the main scanning direction, and are arranged so that the dots formed by main scanning in the forward action or the dots formed by main scanning in the reverse action are contiguous in the main scanning direction; and the proximal pixels are a combination of one pixel and another pixel adjacent to one side of the one pixel in the main scanning direction, and a combination of the one pixel and pixels adjacent to the adjacent pixel on either side in the main scanning direction.
 8. The printing device in claim 1, wherein the printing unit forms dots while performing main scanning in the main scanning direction, and prints the output printed image by performing the main scanning operation a plurality of times; and the first pixel group to which the first pixel belongs and the second pixel group to which the second pixel belongs are groups of pixels in which dots are formed during different main scanning operations among the main scanning operations performed a plurality of times.
 9. The printing device in claim 1, wherein the probability K is within the range k1·k2−0.2<K<k1·k2.
 10. The printing device in claim 1, wherein the predetermined gradation range is 0<k1<0.2, and 0<k2<0.2.
 11. The printing device in claim 1, wherein probabilities k1 and k2 are both k, and probability k is close to k².
 12. A printing method for forming dots in pixels arranged on a printing medium according to an input image data and printing an image to perform printing of an output printed image, the printing method comprising: printing the output printed image by dividing formation of the dots in a common region into a plurality of pixel groups, in a case where first and second pixels belonging, respectively, to two pixel groups among the plurality of pixel groups are proximal pixels in a predetermined gradation range in which probabilities k1 and k2 at which a dot is formed in the first and second pixels are such that k1<0.5 and k2<0.5, a probability K of a dot being formed on both of the proximal pixels is set to be close to k1·k2. 